Johann Sebastian Bach was a prolific composer.
To set the text color of the H1 elements to red, you can write the
following CSS rules:
h1 { color: red }
A CSS rule consists of two main parts: selector ('h1') and declaration
('color: red'). In HTML, element names are case-insensitive so 'h1'
works just as well as 'H1'. The declaration has two parts: property
('color') and value ('red'). While the example above tries to influence
only one of the properties needed for rendering an HTML document, it
qualifies as a style sheet on its own. Combined with other style sheets
(one fundamental feature of CSS is that style sheets are combined) it
will determine the final presentation of the document.
The HTML 4 specification defines how style sheet rules may be specified
for HTML documents: either within the HTML document, or via an external
style sheet. To put the style sheet into the document, use the STYLE
element:
Bach's home page
Bach's home page
Johann Sebastian Bach was a prolific composer.
For maximum flexibility, we recommend that authors specify external
style sheets; they may be changed without modifying the source HTML
document, and they may be shared among several documents. To link to an
external style sheet, you can use the LINK element:
Bach's home page
Bach's home page
Johann Sebastian Bach was a prolific composer.
The LINK element specifies:
* the type of link: to a "stylesheet".
* the location of the style sheet via the "href" attribute.
* the type of style sheet being linked: "text/css".
To show the close relationship between a style sheet and the structured
markup, we continue to use the STYLE element in this tutorial. Let's
add more colors:
Bach's home page
Bach's home page
Johann Sebastian Bach was a prolific composer.
The style sheet now contains four rules: the first two set the color
and background of the BODY element (it's a good idea to set the text
color and background color together), while the last two set the color
and the background of the H1 element. Since no color has been specified
for the P element, it will inherit the color from its parent element,
namely BODY. The H1 element is also a child element of BODY but the
second rule overrides the inherited value. In CSS there are often such
conflicts between different values, and this specification describes
how to resolve them.
CSS 2.1 has more than 90 properties, including 'color'. Let's look at
some of the others:
Bach's home page
Bach's home page
Johann Sebastian Bach was a prolific composer.
The first thing to notice is that several declarations are grouped
within a block enclosed by curly braces ({...}), and separated by
semicolons, though the last declaration may also be followed by a
semicolon.
The first declaration on the BODY element sets the font family to "Gill
Sans". If that font isn't available, the user agent (often referred to
as a "browser") will use the 'sans-serif' font family which is one of
five generic font families which all users agents know. Child elements
of BODY will inherit the value of the 'font-family' property.
The second declaration sets the font size of the BODY element to 12
points. The "point" unit is commonly used in print-based typography to
indicate font sizes and other length values. It's an example of an
absolute unit which does not scale relative to the environment.
The third declaration uses a relative unit which scales with regard to
its surroundings. The "em" unit refers to the font size of the element.
In this case the result is that the margins around the BODY element are
three times wider than the font size.
2.2 A brief CSS 2.1 tutorial for XML
This section is non-normative.
CSS can be used with any structured document format, for example with
applications of the eXtensible Markup Language [XML10]. In fact, XML
depends more on style sheets than HTML, since authors can make up their
own elements that user agents don't know how to display.
Here is a simple XML fragment:
Fredrick the Great meets BachJohann Nikolaus Forkel
One evening, just as he was getting his
flute ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
To display this fragment in a document-like fashion, we must first
declare which elements are inline-level (i.e., do not cause line
breaks) and which are block-level (i.e., cause line breaks).
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
The first rule declares INSTRUMENT to be inline and the second rule,
with its comma-separated list of selectors, declares all the other
elements to be block-level. Element names in XML are case-sensitive, so
a selector written in lowercase (e.g. 'instrument') is different from
uppercase (e.g. 'INSTRUMENT').
One way of linking a style sheet to an XML document is to use a
processing instruction:
Fredrick the Great meets BachJohann Nikolaus Forkel
One evening, just as he was getting his
flute ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
A visual user agent could format the above example as:
Example rendering [D]
Notice that the word "flute" remains within the paragraph since it is
the content of the inline element INSTRUMENT.
Still, the text isn't formatted the way you would expect. For example,
the headline font size should be larger than then the rest of the text,
and you may want to display the author's name in italic:
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }
A visual user agent could format the above example as:
Example rendering [D]
Adding more rules to the style sheet will allow you to further describe
the presentation of the document.
2.3 The CSS 2.1 processing model
This section up to but not including its subsections is non-normative.
This section presents one possible model of how user agents that
support CSS work. This is only a conceptual model; real implementations
may vary.
In this model, a user agent processes a source by going through the
following steps:
1. Parse the source document and create a document tree.
2. Identify the target media type.
3. Retrieve all style sheets associated with the document that are
specified for the target media type.
4. Annotate every element of the document tree by assigning a single
value to every property that is applicable to the target media
type. Properties are assigned values according to the mechanisms
described in the section on cascading and inheritance.
Part of the calculation of values depends on the formatting
algorithm appropriate for the target media type. For example, if
the target medium is the screen, user agents apply the visual
formatting model.
5. From the annotated document tree, generate a formatting structure.
Often, the formatting structure closely resembles the document
tree, but it may also differ significantly, notably when authors
make use of pseudo-elements and generated content. First, the
formatting structure need not be "tree-shaped" at all -- the nature
of the structure depends on the implementation. Second, the
formatting structure may contain more or less information than the
document tree. For instance, if an element in the document tree has
a value of 'none' for the 'display' property, that element will
generate nothing in the formatting structure. A list element, on
the other hand, may generate more information in the formatting
structure: the list element's content and list style information
(e.g., a bullet image).
Note that the CSS user agent does not alter the document tree
during this phase. In particular, content generated due to style
sheets is not fed back to the document language processor (e.g.,
for reparsing).
6. Transfer the formatting structure to the target medium (e.g., print
the results, display them on the screen, render them as speech,
etc.).
2.3.1 The canvas
For all media, the term canvas describes "the space where the
formatting structure is rendered." The canvas is infinite for each
dimension of the space, but rendering generally occurs within a finite
region of the canvas, established by the user agent according to the
target medium. For instance, user agents rendering to a screen
generally impose a minimum width and choose an initial width based on
the dimensions of the viewport. User agents rendering to a page
generally impose width and height constraints. Aural user agents may
impose limits in audio space, but not in time.
2.3.2 CSS 2.1 addressing model
CSS 2.1 selectors and properties allow style sheets to refer to the
following parts of a document or user agent:
* Elements in the document tree and certain relationships between
them (see the section on selectors).
* Attributes of elements in the document tree, and values of those
attributes (see the section on attribute selectors).
* Some parts of element content (see the :first-line and
:first-letter pseudo-elements).
* Elements of the document tree when they are in a certain state (see
the section on pseudo-classes).
* Some aspects of the canvas where the document will be rendered.
* Some system information (see the section on user interface).
2.4 CSS design principles
This section is non-normative.
CSS 2.1, as CSS2 and CSS1 before it, is based on a set of design
principles:
* Forward and backward compatibility. CSS 2.1 user agents will be
able to understand CSS1 style sheets. CSS1 user agents will be able
to read CSS 2.1 style sheets and discard parts they don't
understand. Also, user agents with no CSS support will be able to
display style-enhanced documents. Of course, the stylistic
enhancements made possible by CSS will not be rendered, but all
content will be presented.
* Complementary to structured documents. Style sheets complement
structured documents (e.g., HTML and XML applications), providing
stylistic information for the marked-up text. It should be easy to
change the style sheet with little or no impact on the markup.
* Vendor, platform, and device independence. Style sheets enable
documents to remain vendor, platform, and device independent. Style
sheets themselves are also vendor and platform independent, but
CSS 2.1 allows you to target a style sheet for a group of devices
(e.g., printers).
* Maintainability. By pointing to style sheets from documents,
webmasters can simplify site maintenance and retain consistent look
and feel throughout the site. For example, if the organization's
background color changes, only one file needs to be changed.
* Simplicity. CSS is a simple style language which is human readable
and writable. The CSS properties are kept independent of each other
to the largest extent possible and there is generally only one way
to achieve a certain effect.
* Network performance. CSS provides for compact encodings of how to
present content. Compared to images or audio files, which are often
used by authors to achieve certain rendering effects, style sheets
most often decrease the content size. Also, fewer network
connections have to be opened which further increases network
performance.
* Flexibility. CSS can be applied to content in several ways. The key
feature is the ability to cascade style information specified in
the default (user agent) style sheet, user style sheets, linked
style sheets, the document head, and in attributes for the elements
forming the document body.
* Richness. Providing authors with a rich set of rendering effects
increases the richness of the Web as a medium of expression.
Designers have been longing for functionality commonly found in
desktop publishing and slide-show applications. Some of the
requested rendering effects conflict with device independence, but
CSS 2.1 goes a long way toward granting designers their requests.
* Alternative language bindings. The set of CSS properties described
in this specification form a consistent formatting model for visual
and aural presentations. This formatting model can be accessed
through the CSS language, but bindings to other languages are also
possible. For example, a JavaScript program may dynamically change
the value of a certain element's 'color' property.
* Accessibility. Several CSS features will make the Web more
accessible to users with disabilities:
+ Properties to control font appearance allow authors to
eliminate inaccessible bit-mapped text images.
+ Positioning properties allow authors to eliminate mark-up
tricks (e.g., invisible images) to force layout.
+ The semantics of !important rules mean that users with
particular presentation requirements can override the author's
style sheets.
+ The 'inherit' value for all properties improves cascading
generality and allows for easier and more consistent style
tuning.
+ Improved media support, including media groups and the
braille, embossed, and tty media types, will allow users and
authors to tailor pages to those devices.
Note. For more information about designing accessible documents
using CSS and HTML, see [WAI-PAGEAUTH].
__________________________________________________________________
__________________________________________________________________
3 Conformance: Requirements and Recommendations
Contents
* 3.1 Definitions
* 3.2 UA Conformance
* 3.3 Error conditions
* 3.4 The text/css content type
3.1 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 (see
[RFC2119]). However, for readability, these words do not appear in all
uppercase letters in this specification.
At times, this specification recommends good practice for authors and
user agents. These recommendations are not normative and conformance
with this specification does not depend on their realization. These
recommendations contain the expression "We recommend ...", "This
specification recommends ...", or some similar wording.
The fact that a feature is marked as deprecated (namely the 'aural'
keyword) or going to be deprecated in CSS3 (namely the system colors)
also has no influence on conformance. (For example, 'aural' is marked
as non-normative, so UAs do not need to support it; the system colors
are normative, so UAs must support them.)
All sections of this specification, including appendices, are normative
unless otherwise noted.
Examples and notes are not normative.
Example(s):
Examples usually have the word "example" near their start ("Example:",
"The following example...," "For example," etc.) and are shown in the
color maroon, like this paragraph.
Notes start with the word "Note," are indented and shown in green, like
this paragraph.
Figures are for illustration only, they are not reference renderings,
unless explicitly stated.
Style sheet
A set of statements that specify presentation of a document.
Style sheets may have three different origins: author, user, and
user agent. The interaction of these sources is described in the
section on cascading and inheritance.
Valid style sheet
The validity of a style sheet depends on the level of CSS used
for the style sheet. All valid CSS1 style sheets are valid
CSS 2.1 style sheets, but some changes from CSS1 mean that a few
CSS1 style sheets will have slightly different semantics in
CSS 2.1. Some features in CSS2 are not part of CSS 2.1, so not
all CSS2 style sheets are valid CSS 2.1 style sheets.
A valid CSS 2.1 style sheet must be written according to the
grammar of CSS 2.1. Furthermore, it must contain only at-rules,
property names, and property values defined in this
specification. An illegal (invalid) at-rule, property name, or
property value is one that is not valid.
Source document
The document to which one or more style sheets apply. This is
encoded in some language that represents the document as a tree
of elements. Each element consists of a name that identifies the
type of element, optionally a number of attributes, and a
(possibly empty) content. For example, the source document could
be an XML or SGML instance.
Document language
The encoding language of the source document (e.g., HTML, XHTML
or SVG). CSS is used to describe the presentation of document
languages and CSS does not change the underlying semantics of
the document languages.
Element
(An SGML term, see [ISO8879].) The primary syntactic constructs
of the document language. Most CSS style sheet rules use the
names of these elements (such as P, TABLE, and OL in HTML) to
specify how the elements should be rendered.
Replaced element
An element whose content is outside the scope of the CSS
formatting model, such as an image, embedded document, or
applet. For example, the content of the HTML IMG element is
often replaced by the image that its "src" attribute designates.
Replaced elements often have intrinsic dimensions: an intrinsic
width, an intrinsic height, and an intrinsic ratio. For example,
a bitmap image has an intrinsic width and an intrinsic height
specified in absolute units (from which the intrinsic ratio can
obviously be determined). On the other hand, other documents may
not have any intrinsic dimensions (for example a blank HTML
document).
User agents may consider a replaced element to not have any
intrinsic dimensions if it is believed that those dimensions
could leak sensitive information to a third party. For example,
if an HTML document changed intrinsic size depending on the
user's bank balance, then the UA might want to act as if that
resource had no intrinsic dimensions.
Intrinsic dimensions
The width and height as defined by the element itself, not
imposed by the surroundings. CSS does not define how the
intrinsic dimensions are found. In CSS 2.1 only replaced
elements can come with intrinsic dimensions. For raster images
without reliable resolution information, a size of 1 px unit per
image source pixel must be assumed.
Attribute
A value associated with an element, consisting of a name, and an
associated (textual) value.
Content
The content associated with an element in the source document.
Some elements have no content, in which case they are called
empty. The content of an element may include text, and it may
include a number of sub-elements, in which case the element is
called the parent of those sub-elements.
Ignore
This term has two slightly different meanings in this
specification. First, a CSS parser must follow certain rules
when it discovers unknown or illegal syntax in a style sheet.
The parser must then ignore certain parts of the style sheets.
The exact rules for what parts must be ignored is given in these
section: Declarations and properties, Rules for handling parsing
errors, Unsupported Values, or may be explained in the text
where the term "ignore" appears. Second, a user agent may (and,
in some cases must) disregard certain properties or values in
the style sheet even if the syntax is legal. For example,
table-column elements can't affect the font of the column, so
the font properties must be ignored.
Rendered content
The content of an element after the rendering that applies to it
according to the relevant style sheets has been applied. How a
replaced element's content is rendered is not defined by this
specification. Rendered content may also be alternate text for
an element (e.g., the value of the XHTML "alt" attribute), and
may include items inserted implicitly or explicitly by the style
sheet, such as bullets, numbering, etc.
Document tree
The tree of elements encoded in the source document. Each
element in this tree has exactly one parent, with the exception
of the root element, which has none.
Child
An element A is called the child of element B if and only if B
is the parent of A.
Descendant
An element A is called a descendant of an element B, if either
(1) A is a child of B, or (2) A is the child of some element C
that is a descendant of B.
Ancestor
An element A is called an ancestor of an element B, if and only
if B is a descendant of A.
Sibling
An element A is called a sibling of an element B, if and only if
B and A share the same parent element. Element A is a preceding
sibling if it comes before B in the document tree. Element B is
a following sibling if it comes after A in the document tree.
Preceding element
An element A is called a preceding element of an element B, if
and only if (1) A is an ancestor of B or (2) A is a preceding
sibling of B.
Following element
An element A is called a following element of an element B, if
and only if B is a preceding element of A.
Author
An author is a person who writes documents and associated style
sheets. An authoring tool is a User Agent that generates style
sheets.
User
A user is a person who interacts with a user agent to view,
hear, or otherwise use a document and its associated style
sheet. The user may provide a personal style sheet that encodes
personal preferences.
User agent (UA)
A user agent is any program that interprets a document written
in the document language and applies associated style sheets
according to the terms of this specification. A user agent may
display a document, read it aloud, cause it to be printed,
convert it to another format, etc.
An HTML user agent is one that supports the HTML 2.x, HTML 3.x,
or HTML 4.x specifications. A user agent that supports XHTML
[XHTML], but not HTML (as listed in the previous sentence) is
not considered an HTML user agent for the purpose of conformance
with this specification.
Property
CSS defines a finite set of parameters, called properties, that
direct the rendering of a document. Each property has a name
(e.g., 'color', 'font' or border') and a value (e.g., 'red',
'12pt Times' or 'dotted'). Properties are attached to various
parts of the document and to the page on which the document is
to be displayed by the mechanisms of specificity, cascading and
inheritance (see the chapter on Assigning property values,
Cascading, and Inheritance).
Here is an example of a source document written in HTML:
My home page
My home page
Welcome to my home page! Let me tell you about my favorite
composers:
Elvis Costello
Johannes Brahms
Georges Brassens
This results in the following tree:
Sample document tree [D]
According to the definition of HTML 4, HEAD elements will be inferred
during parsing and become part of the document tree even if the "head"
tags are not in the document source. Similarly, the parser knows where
the P and LI elements end, even though there are no
and tags
in the source.
Documents written in XHTML (and other XML-based languages) behave
differently: there are no inferred elements and all elements must have
end tags.
3.2 UA Conformance
This section defines conformance with the CSS 2.1 specification only.
There may be other levels of CSS in the future that may require a user
agent to implement a different set of features in order to conform.
In general, the following points must be observed by a user agent
claiming conformance to this specification:
1. It must recognize one or more of the CSS 2.1 media types.
2. For each source document, it must attempt to retrieve all
associated style sheets that are appropriate for the recognized
media types. If it cannot retrieve all associated style sheets (for
instance, because of network errors), it must display the document
using those it can retrieve.
3. It must parse the style sheets according to this specification. In
particular, it must recognize all at-rules, blocks, declarations,
and selectors (see the grammar of CSS 2.1). If a user agent
encounters a property that applies for a supported media type, the
user agent must parse the value according to the property
definition. This means that the user agent must accept all valid
values and must ignore declarations with invalid values. User
agents must ignore rules that apply to unsupported media types.
4. For each element in a document tree, it must assign a value for
every property according to the property's definition and the rules
of cascading and inheritance.
5. If the source document comes with alternate style sheet sets (such
as with the "alternate" keyword in HTML 4 [HTML4]), the UA must
allow the user to select which style sheet set the UA should apply.
6. The UA must allow the user to turn off the influence of author
style sheets.
Not every user agent must observe every point, however:
* An application that reads style sheets without rendering any
content (e.g., a CSS 2.1 validator) must respect points 1-3.
* An authoring tool is only required to output valid style sheets
* A user agent that renders a document with associated style sheets
must respect points 1-6 and render the document according to the
media-specific requirements set forth in this specification. Values
may be approximated when required by the user agent.
The inability of a user agent to implement part of this specification
due to the limitations of a particular device (e.g., a user agent
cannot render colors on a monochrome monitor or page) does not imply
non-conformance.
UAs must allow users to specify a file that contains the user style
sheet. UAs that run on devices without any means of writing or
specifying files are exempted from this requirement. Additionally, UAs
may offer other means to specify user preferences, for example through
a GUI.
CSS 2.1 does not define which properties apply to form controls and
frames, or how CSS can be used to style them. User agents may apply CSS
properties to these elements. Authors are recommended to treat such
support as experimental. A future level of CSS may specify this
further.
3.3 Error conditions
In general, this document specifies error handling behavior throughout
the specification. For example, see the rules for handling parsing
errors.
3.4 The text/css content type
CSS style sheets that exist in separate files are sent over the
Internet as a sequence of bytes accompanied by encoding information.
The structure of the transmission, termed a message entity, is defined
by RFC 2045 and RFC 2616 (see [RFC2045] and [RFC2616]). A message
entity with a content type of "text/css" represents an independent CSS
document. The "text/css" content type has been registered by RFC 2318
([RFC2318]).
__________________________________________________________________
__________________________________________________________________
4 Syntax and basic data types
Contents
* 4.1 Syntax
+ 4.1.1 Tokenization
+ 4.1.2 Keywords
o 4.1.2.1 Vendor-specific extensions
o 4.1.2.2 Informative Historical Notes
+ 4.1.3 Characters and case
+ 4.1.4 Statements
+ 4.1.5 At-rules
+ 4.1.6 Blocks
+ 4.1.7 Rule sets, declaration blocks, and selectors
+ 4.1.8 Declarations and properties
+ 4.1.9 Comments
* 4.2 Rules for handling parsing errors
* 4.3 Values
+ 4.3.1 Integers and real numbers
+ 4.3.2 Lengths
+ 4.3.3 Percentages
+ 4.3.4 URLs and URIs
+ 4.3.5 Counters
+ 4.3.6 Colors
+ 4.3.7 Strings
+ 4.3.8 Unsupported Values
* 4.4 CSS style sheet representation
+ 4.4.1 Referring to characters not represented in a character
encoding
4.1 Syntax
This section describes a grammar (and forward-compatible parsing rules)
common to any level of CSS (including CSS 2.1). Future updates of CSS
will adhere to this core syntax, although they may add additional
syntactic constraints.
These descriptions are normative. They are also complemented by the
normative grammar rules presented in Appendix G.
In this specification, the expressions "immediately before" or
"immediately after" mean with no intervening white space or comments.
4.1.1 Tokenization
All levels of CSS -- level 1, level 2, and any future levels -- use the
same core syntax. This allows UAs to parse (though not completely
understand) style sheets written in levels of CSS that didn't exist at
the time the UAs were created. Designers can use this feature to create
style sheets that work with older user agents, while also exercising
the possibilities of the latest levels of CSS.
At the lexical level, CSS style sheets consist of a sequence of tokens.
The list of tokens for CSS is as follows. The definitions use Lex-style
regular expressions. Octal codes refer to ISO 10646 ([ISO10646]). As in
Lex, in case of multiple matches, the longest match determines the
token.
Token Definition
__________________________________________________________________
IDENT {ident}
ATKEYWORD @{ident}
STRING {string}
INVALID {invalid}
HASH #{name}
NUMBER {num}
PERCENTAGE {num}%
DIMENSION {num}{ident}
URI url\({w}{string}{w}\)
|url\({w}([!#$%&*-~]|{nonascii}|{escape})*{w}\)
UNICODE-RANGE u\+[0-9a-f?]{1,6}(-[0-9a-f]{1,6})?
CDO
; ;
{ \{
} \}
( \(
) \)
[ \[
] \]
S [ \t\r\n\f]+
COMMENT \/\*[^*]*\*+([^/*][^*]*\*+)*\/
FUNCTION {ident}\(
INCLUDES ~=
DASHMATCH |=
DELIM any other character not matched by the above rules, and neither a
single nor a double quote
The macros in curly braces ({}) above are defined as follows:
Macro Definition
__________________________________________________________________
ident [-]?{nmstart}{nmchar}*
name {nmchar}+
nmstart [_a-z]|{nonascii}|{escape}
nonascii [^\0-\177]
unicode \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
escape {unicode}|\\[^\n\r\f0-9a-f]
nmchar [_a-z0-9-]|{nonascii}|{escape}
num [0-9]+|[0-9]*\.[0-9]+
string {string1}|{string2}
string1 \"([^\n\r\f\\"]|\\{nl}|{escape})*\"
string2 \'([^\n\r\f\\']|\\{nl}|{escape})*\'
invalid {invalid1}|{invalid2}
invalid1 \"([^\n\r\f\\"]|\\{nl}|{escape})*
invalid2 \'([^\n\r\f\\']|\\{nl}|{escape})*
nl \n|\r\n|\r|\f
w [ \t\r\n\f]*
Below is the core syntax for CSS. The sections that follow describe how
to use it. Appendix G describes a more restrictive grammar that is
closer to the CSS level 2 language. Parts of style sheets that can be
parsed according to this grammar but not according to the grammar in
Appendix G are among the parts that will be ignored according to the
rules for handling parsing errors.
stylesheet : [ CDO | CDC | S | statement ]*;
statement : ruleset | at-rule;
at-rule : ATKEYWORD S* any* [ block | ';' S* ];
block : '{' S* [ any | block | ATKEYWORD S* | ';' S* ]* '}' S*;
ruleset : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector : any+;
declaration : property S* ':' S* value;
property : IDENT;
value : [ any | block | ATKEYWORD S* ]+;
any : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
| DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
| DASHMATCH | FUNCTION S* any* ')'
| '(' S* any* ')' | '[' S* any* ']' ] S*;
COMMENT tokens do not occur in the grammar (to keep it readable), but
any number of these tokens may appear anywhere outside other tokens.
(Note, however, that a comment before or within the @charset rule
disables the @charset.)
The token S in the grammar above stands for white space. Only the
characters "space" (U+0020), "tab" (U+0009), "line feed" (U+000A),
"carriage return" (U+000D), and "form feed" (U+000C) can occur in white
space. Other space-like characters, such as "em-space" (U+2003) and
"ideographic space" (U+3000), are never part of white space.
The meaning of input that cannot be tokenized or parsed is undefined in
CSS 2.1.
4.1.2 Keywords
Keywords have the form of identifiers. Keywords must not be placed
between quotes ("..." or '...'). Thus,
red
is a keyword, but
"red"
is not. (It is a string.) Other illegal examples:
Illegal example(s):
width: "auto";
border: "none";
background: "red";
4.1.2.1 Vendor-specific extensions
In CSS, identifiers may begin with '-' (dash) or '_' (underscore).
Keywords and property names beginning with -' or '_' are reserved for
vendor-specific extensions. Such vendor-specific extensions should have
one of the following formats:
'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name
Example(s):
For example, if XYZ organization added a property to describe the color
of the border on the East side of the display, they might call it
-xyz-border-east-color.
Other known examples:
-moz-box-sizing
-moz-border-radius
-wap-accesskey
An initial dash or underscore is guaranteed never to be used in a
property or keyword by any current or future level of CSS. Thus typical
CSS implementations may not recognize such properties and may ignore
them according to the rules for handling parsing errors. However,
because the initial dash or underscore is part of the grammar, CSS 2.1
implementers should always be able to use a CSS-conforming parser,
whether or not they support any vendor-specific extensions.
Authors should avoid vendor-specific extensions
4.1.2.2 Informative Historical Notes
This section is informative.
At the time of writing, the following prefixes are known to exist:
prefix organization
-ms-, mso- Microsoft
-moz- Mozilla
-o-, -xv- Opera Software
-atsc- Advanced Television Standards Committee
-wap- The WAP Forum
-khtml- KDE
-webkit- Apple
prince- YesLogic
-ah- Antenna House
-hp- Hewlett Packard
-ro- Real Objects
-rim- Research In Motion
4.1.3 Characters and case
The following rules always hold:
* All CSS syntax is case-insensitive within the ASCII range (i.e.
[a-z] and [A-Z] are equivalent), except for parts that are not
under the control of CSS. For example, the case-sensitivity of
values of the HTML attributes "id" and "class", of font names, and
of URIs lies outside the scope of this specification. Note in
particular that element names are case-insensitive in HTML, but
case-sensitive in XML.
* In CSS, identifiers (including element names, classes, and IDs in
selectors) can contain only the characters [a-zA-Z0-9] and ISO
10646 characters U+00A1 and higher, plus the hyphen (-) and the
underscore (_); they cannot start with a digit, or a hyphen
followed by a digit. Identifiers can also contain escaped
characters and any ISO 10646 character as a numeric code (see next
item). For instance, the identifier "B&W?" may be written as
"B\&W\?" or "B\26 W\3F".
Note that Unicode is code-by-code equivalent to ISO 10646 (see
[UNICODE] and [ISO10646]).
* In CSS 2.1, a backslash (\) character indicates three types of
character escapes.
First, inside a string, a backslash followed by a newline is
ignored (i.e., the string is deemed not to contain either the
backslash or the newline).
Second, it cancels the meaning of special CSS characters. Except
within CSS comments, any character (except a hexadecimal digit,
linefeed, carriage return or form feed) can be escaped with a
backslash to remove its special meaning. For example, "\"" is a
string consisting of one double quote. Style sheet preprocessors
must not remove these backslashes from a style sheet since that
would change the style sheet's meaning.
Third, backslash escapes allow authors to refer to characters they
can't easily put in a document. In this case, the backslash is
followed by at most six hexadecimal digits (0..9A..F), which stand
for the ISO 10646 ([ISO10646]) character with that number, which
must not be zero. (It is undefined in CSS 2.1 what happens if a
style sheet does contain a character with Unicode codepoint zero.)
If a character in the range [0-9a-fA-F] follows the hexadecimal
number, the end of the number needs to be made clear. There are two
ways to do that:
1. with a space (or other white space character): "\26 B" ("&B").
In this case, user agents should treat a "CR/LF" pair
(U+000D/U+000A) as a single white space character.
2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")
In fact, these two methods may be combined. Only one white space
character is ignored after a hexadecimal escape. Note that this
means that a "real" space after the escape sequence must itself
either be escaped or doubled.
If the number is outside the range allowed by Unicode (e.g.,
"\110000" is above the maximum 10FFFF allowed in current Unicode),
the UA may replace the escape with the "replacement character"
(U+FFFD). If the character is to be displayed, the UA should show a
visible symbol, such as a "missing character" glyph (cf. 15.2,
point 5).
* Note: Backslash escapes, where allowed, are always considered to be
part of an identifier or a string (i.e., "\7B" is not punctuation,
even though "{" is, and "\32" is allowed at the start of a class
name, even though "2" is not).
The identifier "te\st" is exactly the same identifier as "test".
4.1.4 Statements
A CSS style sheet, for any level of CSS, consists of a list of
statements (see the grammar above). There are two kinds of statements:
at-rules and rule sets. There may be white space around the statements.
4.1.5 At-rules
At-rules start with an at-keyword, an '@' character followed
immediately by an identifier (for example, '@import', '@page').
An at-rule consists of everything up to and including the next
semicolon (;) or the next block, whichever comes first.
CSS 2.1 user agents must ignore any '@import' rule that occurs inside a
block or after any non-ignored statement other than an @charset or an
@import rule.
Illegal example(s):
Assume, for example, that a CSS 2.1 parser encounters this style sheet:
@import "subs.css";
h1 { color: blue }
@import "list.css";
The second '@import' is illegal according to CSS 2.1. The CSS 2.1
parser ignores the whole at-rule, effectively reducing the style sheet
to:
@import "subs.css";
h1 { color: blue }
Illegal example(s):
In the following example, the second '@import' rule is invalid, since
it occurs inside a '@media' block.
@import "subs.css";
@media print {
@import "print-main.css";
body { font-size: 10pt }
}
h1 {color: blue }
Instead, to achieve the effect of only importing a style sheet for
'print' media, use the @import rule with media syntax, e.g.:
@import "subs.css";
@import "print-main.css" print;
@media print {
body { font-size: 10pt }
}
h1 {color: blue }
4.1.6 Blocks
A block starts with a left curly brace ({) and ends with the matching
right curly brace (}). In between there may be any tokens, except that
parentheses (( )), brackets ([ ]) and braces ({ }) must always occur in
matching pairs and may be nested. Single (') and double quotes (") must
also occur in matching pairs, and characters between them are parsed as
a string. See Tokenization above for the definition of a string.
Illegal example(s):
Here is an example of a block. Note that the right brace between the
double quotes does not match the opening brace of the block, and that
the second single quote is an escaped character, and thus doesn't match
the first single quote:
{ causta: "}" + ({7} * '\'') }
Note that the above rule is not valid CSS 2.1, but it is still a block
as defined above.
4.1.7 Rule sets, declaration blocks, and selectors
A rule set (also called "rule") consists of a selector followed by a
declaration block.
A declaration block starts with a left curly brace ({) and ends with
the matching right curly brace (}). In between there must be a list of
zero or more semicolon-separated (;) declarations.
The selector (see also the section on selectors) consists of everything
up to (but not including) the first left curly brace ({). A selector
always goes together with a declaration block. When a user agent can't
parse the selector (i.e., it is not valid CSS 2.1), it must ignore the
selector and the following declaration block (if any) as well.
CSS 2.1 gives a special meaning to the comma (,) in selectors. However,
since it is not known if the comma may acquire other meanings in future
updates of CSS, the whole statement should be ignored if there is an
error anywhere in the selector, even though the rest of the selector
may look reasonable in CSS 2.1.
Illegal example(s):
For example, since the "&" is not a valid token in a CSS 2.1 selector,
a CSS 2.1 user agent must ignore the whole second line, and not set the
color of H3 to red:
h1, h2 {color: green }
h3, h4 & h5 {color: red }
h6 {color: black }
Example(s):
Here is a more complex example. The first two pairs of curly braces are
inside a string, and do not mark the end of the selector. This is a
valid CSS 2.1 rule.
p[example="public class foo\
{\
private int x;\
\
foo(int x) {\
this.x = x;\
}\
\
}"] { color: red }
4.1.8 Declarations and properties
A declaration is either empty or consists of a property name, followed
by a colon (:), followed by a value. Around each of these there may be
white space.
Because of the way selectors work, multiple declarations for the same
selector may be organized into semicolon (;) separated groups.
Example(s):
Thus, the following rules:
h1 { font-weight: bold }
h1 { font-size: 12px }
h1 { line-height: 14px }
h1 { font-family: Helvetica }
h1 { font-variant: normal }
h1 { font-style: normal }
are equivalent to:
h1 {
font-weight: bold;
font-size: 12px;
line-height: 14px;
font-family: Helvetica;
font-variant: normal;
font-style: normal
}
A property name is an identifier. Any token may occur in the value.
Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes
(') and double quotes (") must come in matching pairs, and semicolons
not in strings must be escaped. Parentheses, brackets, and braces may
be nested. Inside the quotes, characters are parsed as a string.
The syntax of values is specified separately for each property, but in
any case, values are built from identifiers, strings, numbers, lengths,
percentages, URIs, colors, etc.
A user agent must ignore a declaration with an invalid property name or
an invalid value. Every CSS 2.1 property has its own syntactic and
semantic restrictions on the values it accepts.
Illegal example(s):
For example, assume a CSS 2.1 parser encounters this style sheet:
h1 { color: red; font-style: 12pt } /* Invalid value: 12pt */
p { color: blue; font-vendor: any; /* Invalid prop.: font-vendor */
font-variant: small-caps }
em em { font-style: normal }
The second declaration on the first line has an invalid value '12pt'.
The second declaration on the second line contains an undefined
property 'font-vendor'. The CSS 2.1 parser will ignore these
declarations, effectively reducing the style sheet to:
h1 { color: red; }
p { color: blue; font-variant: small-caps }
em em { font-style: normal }
4.1.9 Comments
Comments begin with the characters "/*" and end with the characters
"*/". They may occur anywhere between tokens, and their contents have
no influence on the rendering. Comments may not be nested.
CSS also allows the SGML comment delimiters ("") in
certain places defined by the grammar, but they do not delimit CSS
comments. They are permitted so that style rules appearing in an HTML
source document (in the STYLE element) may be hidden from pre-HTML 3.2
user agents. See the HTML 4 specification ([HTML4]) for more
information.
4.2 Rules for handling parsing errors
In some cases, user agents must ignore part of an illegal style sheet.
This specification defines ignore to mean that the user agent parses
the illegal part (in order to find its beginning and end), but
otherwise acts as if it had not been there. CSS 2.1 reserves for future
updates of CSS all property:value combinations and @-keywords that do
not contain an identifier beginning with dash or underscore.
Implementations must ignore such combinations (other than those
introduced by future updates of CSS).
To ensure that new properties and new values for existing properties
can be added in the future, user agents are required to obey the
following rules when they encounter the following scenarios:
* Unknown properties. User agents must ignore a declaration with an
unknown property. For example, if the style sheet is:
h1 { color: red; rotation: 70minutes }
the user agent will treat this as if the style sheet had been
h1 { color: red }
* Illegal values. User agents must ignore a declaration with an
illegal value. For example:
img { float: left } /* correct CSS 2.1 */
img { float: left here } /* "here" is not a value of 'float' */
img { background: "red" } /* keywords cannot be quoted */
img { border-width: 3 } /* a unit must be specified for length values */
A CSS 2.1 parser would honor the first rule and ignore the rest, as
if the style sheet had been:
img { float: left }
img { }
img { }
img { }
A user agent conforming to a future CSS specification may accept
one or more of the other rules as well.
* Malformed declarations. User agents must handle unexpected tokens
encountered while parsing a declaration by reading until the end of
the declaration, while observing the rules for matching pairs of
(), [], {}, "", and '', and correctly handling escapes. For
example, a malformed declaration may be missing a property, colon
(:) or value. The following are all equivalent:
p { color:green }
p { color:green; color } /* malformed declaration missing ':', value */
p { color:red; color; color:green } /* same with expected recovery */
p { color:green; color: } /* malformed declaration missing value */
p { color:red; color:; color:green } /* same with expected recovery */
p { color:green; color{;color:maroon} } /* unexpected tokens { } */
p { color:red; color{;color:maroon}; color:green } /* same with recovery */
* Malformed statements. User agents must handle unexpected tokens
encountered while parsing a statement by reading until the end of
the statement, while observing the rules for matching pairs of (),
[], {}, "", and '', and correctly handling escapes. For example, a
malformed statement may contain an unexpected closing brace or
at-keyword. E.g., the following lines are all ignored:
p @here {color: red} /* ruleset with unexpected at-keyword "@here" */
@foo @bar; /* at-rule with unexpected at-keyword "@bar" */
}} {{ - }} /* ruleset with unexpected right brace */
) ( {} ) p {color: red } /* ruleset with unexpected right parenthesis */
* Invalid at-keywords. User agents must ignore an invalid at-keyword
together with everything following it, up to and including the next
semicolon (;), the next block ({...}), or the end of the block (})
that contains the invalid at-keyword, whichever comes first. For
example, consider the following:
@three-dee {
@background-lighting {
azimuth: 30deg;
elevation: 190deg;
}
h1 { color: red }
}
h1 { color: blue }
The '@three-dee' at-rule is not part of CSS 2.1. Therefore, the
whole at-rule (up to, and including, the third right curly brace)
is ignored. A CSS 2.1 user agent ignores it, effectively reducing
the style sheet to:
h1 { color: blue }
Something inside an at-rule that is ignored because it is invalid,
such as an invalid declaration within an @media-rule, does not make
the entire at-rule invalid.
* Unexpected end of style sheet.
User agents must close all open constructs (for example: blocks,
parentheses, brackets, rules, strings, and comments) at the end of
the style sheet. For example:
@media screen {
p:before { content: 'Hello
would be treated the same as:
@media screen {
p:before { content: 'Hello'; }
}
in a conformant UA.
* Unexpected end of string.
User agents must close strings upon reaching the end of a line, but
then drop the construct (declaration or rule) in which the string
was found. For example:
p {
color: green;
font-family: 'Courier New Times
color: red;
color: green;
}
...would be treated the same as:
p { color: green; color: green; }
...because the second declaration (from 'font-family' to the
semicolon after 'color: red') is invalid and is dropped.
* See also Rule sets, declaration blocks, and selectors for parsing
rules for declaration blocks.
4.3 Values
4.3.1 Integers and real numbers
Some value types may have integer values (denoted by ) or real
number values (denoted by ). Real numbers and integers are
specified in decimal notation only. An consists of one or
more digits "0" to "9". A can either be an , or it
can be zero or more digits followed by a dot (.) followed by one or
more digits. Both integers and real numbers may be preceded by a "-" or
"+" to indicate the sign. -0 is equivalent to 0 and is not a negative
number.
Note that many properties that allow an integer or real number as a
value actually restrict the value to some range, often to a
non-negative value.
4.3.2 Lengths
Lengths refer to horizontal or vertical measurements.
The format of a length value (denoted by in this
specification) is a (with or without a decimal point)
immediately followed by a unit identifier (e.g., px, em, etc.). After a
zero length, the unit identifier is optional.
Some properties allow negative length values, but this may complicate
the formatting model and there may be implementation-specific limits.
If a negative length value cannot be supported, it should be converted
to the nearest value that can be supported.
If a negative length value is set on a property that does not allow
negative length values, the declaration is ignored.
There are two types of length units: relative and absolute. Relative
length units specify a length relative to another length property.
Style sheets that use relative units will more easily scale from one
medium to another (e.g., from a computer display to a laser printer).
Relative units are:
* em: the 'font-size' of the relevant font
* ex: the 'x-height' of the relevant font
* px: pixels, relative to the viewing device
Example(s):
h1 { margin: 0.5em } /* em */
h1 { margin: 1ex } /* ex */
p { font-size: 12px } /* px */
The 'em' unit is equal to the computed value of the 'font-size'
property of the element on which it is used. The exception is when 'em'
occurs in the value of the 'font-size' property itself, in which case
it refers to the font size of the parent element. It may be used for
vertical or horizontal measurement. (This unit is also sometimes called
the quad-width in typographic texts.)
The 'ex' unit is defined by the element's first available font. The
'x-height' is so called because it is often equal to the height of the
lowercase "x". However, an 'ex' is defined even for fonts that don't
contain an "x".
The x-height of a font can be found in different ways. Some fonts
contain reliable metrics for the x-height. If reliable font metrics are
not available, UAs may determine the x-height from the height of a
lowercase glyph. One possible heuristics is to look at how far the
glyph for the lowercase "o" extends below the baseline, and subtract
that value from the top of its bounding box. In the cases where it is
impossible or impractical to determine the x-height, a value of 0.5em
should be used.
Example(s):
The rule:
h1 { line-height: 1.2em }
means that the line height of "h1" elements will be 20% greater than
the font size of the "h1" elements. On the other hand:
h1 { font-size: 1.2em }
means that the font-size of "h1" elements will be 20% greater than the
font size inherited by "h1" elements.
When specified for the root of the document tree (e.g., "HTML" in
HTML), 'em' and 'ex' refer to the property's initial value.
Pixel units are relative to the resolution of the viewing device, i.e.,
most often a computer display. If the pixel density of the output
device is very different from that of a typical computer display, the
user agent should rescale pixel values. It is recommended that the
pixel unit refer to the whole number of device pixels that best
approximates the reference pixel. It is recommended that the reference
pixel be the visual angle of one pixel on a device with a pixel density
of 96dpi and a distance from the reader of an arm's length. For a
nominal arm's length of 28 inches, the visual angle is therefore about
0.0213 degrees.
For reading at arm's length, 1px thus corresponds to about 0.26 mm
(1/96 inch). When printed on a laser printer, meant for reading at a
little less than arm's length (55 cm, 21 inches), 1px is about 0.20 mm.
On a 300 dots-per-inch (dpi) printer, that may be rounded up to 3 dots
(0.25 mm); on a 600 dpi printer, it can be rounded to 5 dots.
The two images below illustrate the effect of viewing distance on the
size of a pixel and the effect of a device's resolution. In the first
image, a reading distance of 71 cm (28 inch) results in a px of
0.26 mm, while a reading distance of 3.5 m (12 feet) requires a px of
1.3 mm.
Showing that pixels must become larger if the viewing distance
increases [D]
In the second image, an area of 1px by 1px is covered by a single dot
in a low-resolution device (a computer screen), while the same area is
covered by 16 dots in a higher resolution device (such as a 400 dpi
laser printer).
Showing that more device pixels (dots) are needed to cover a 1px by 1px
area on a high-resolution device than on a low-res one [D]
Child elements do not inherit the relative values specified for their
parent; they inherit the computed values.
Example(s):
In the following rules, the computed 'text-indent' value of "h1"
elements will be 36px, not 45px, if "h1" is a child of the "body"
element.
body {
font-size: 12px;
text-indent: 3em; /* i.e., 36px */
}
h1 { font-size: 15px }
Absolute length units are only useful when the physical properties of
the output medium are known. The absolute units are:
* in: inches -- 1 inch is equal to 2.54 centimeters.
* cm: centimeters
* mm: millimeters
* pt: points -- the points used by CSS 2.1 are equal to 1/72nd of an
inch.
* pc: picas -- 1 pica is equal to 12 points.
Example(s):
h1 { margin: 0.5in } /* inches */
h2 { line-height: 3cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { font-size: 12pt } /* points */
h4 { font-size: 1pc } /* picas */
In cases where the used length cannot be supported, user agents must
approximate it in the actual value.
4.3.3 Percentages
The format of a percentage value (denoted by in this
specification) is a immediately followed by '%'.
Percentage values are always relative to another value, for example a
length. Each property that allows percentages also defines the value to
which the percentage refers. The value may be that of another property
for the same element, a property for an ancestor element, or a value of
the formatting context (e.g., the width of a containing block). When a
percentage value is set for a property of the root element and the
percentage is defined as referring to the inherited value of some
property, the resultant value is the percentage times the initial value
of that property.
Example(s):
Since child elements (generally) inherit the computed values of their
parent, in the following example, the children of the P element will
inherit a value of 12px for 'line-height', not the percentage value
(120%):
p { font-size: 10px }
p { line-height: 120% } /* 120% of 'font-size' */
4.3.4 URLs and URIs
URI values (Uniform Resource Identifiers, see [RFC3986], which includes
URLs, URNs, etc) in this specification are denoted by . The
functional notation used to designate URIs in property values is
"url()", as in:
Example(s):
body { background: url("http://www.example.com/pinkish.png") }
The format of a URI value is 'url(' followed by optional white space
followed by an optional single quote (') or double quote (") character
followed by the URI itself, followed by an optional single quote (') or
double quote (") character followed by optional white space followed by
')'. The two quote characters must be the same.
Example(s):
An example without quotes:
li { list-style: url(http://www.example.com/redball.png) disc }
Some characters appearing in an unquoted URI, such as parentheses,
commas, white space characters, single quotes (') and double quotes
("), must be escaped with a backslash so that the resulting URI value
is a URI token: '\(', '\)', '\,'.
Depending on the type of URI, it might also be possible to write the
above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as
described in [RFC3986].
In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors may use relative URIs.
Relative URIs (as defined in [RFC3986]) are resolved to full URIs using
a base URI. RFC 3986, section 5, defines the normative algorithm for
this process. For CSS style sheets, the base URI is that of the style
sheet, not that of the source document.
Example(s):
For example, suppose the following rule:
body { background: url("yellow") }
is located in a style sheet designated by the URI:
http://www.example.org/style/basic.css
The background of the source document's BODY will be tiled with
whatever image is described by the resource designated by the URI
http://www.example.org/style/yellow
User agents may vary in how they handle invalid URIs or URIs that
designate unavailable or inapplicable resources.
4.3.5 Counters
Counters are denoted by case-sensitive identifiers (see the
'counter-increment' and 'counter-reset' properties). To refer to the
value of a counter, the notation 'counter()' or
'counter(, <'list-style-type'>)', with optional white space
separating the tokens, is used. The default style is 'decimal'.
To refer to a sequence of nested counters of the same name, the
notation is 'counters(, )' or
'counters(, , <'list-style-type'>)' with optional
white space separating the tokens.
See "Nested counters and scope" in the chapter on generated content for
how user agents must determine the value or values of the counter. See
the definition of counter values of the 'content' property for how it
must convert these values to a string.
In CSS 2.1, the values of counters can only be referred to from the
'content' property. Note that 'none' is a possible <'list-style-type'>:
'counter(x, none)' yields an empty string.
Example(s):
Here is a style sheet that numbers paragraphs (p) for each chapter
(h1). The paragraphs are numbered with roman numerals, followed by a
period and a space:
p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}
4.3.6 Colors
A is either a keyword or a numerical RGB specification.
The list of color keywords is: aqua, black, blue, fuchsia, gray, green,
lime, maroon, navy, olive, orange, purple, red, silver, teal, white,
and yellow. These 17 colors have the following values:
maroon #800000 red #ff0000 orange #ffA500 yellow #ffff00 olive #808000
purple #800080 fuchsia #ff00ff white #ffffff lime #00ff00 green #008000
navy #000080 blue #0000ff aqua #00ffff teal #008080
black #000000 silver #c0c0c0 gray #808080
In addition to these color keywords, users may specify keywords that
correspond to the colors used by certain objects in the user's
environment. Please consult the section on system colors for more
information.
Example(s):
body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }
The RGB color model is used in numerical color specifications. These
examples all specify the same color:
Example(s):
em { color: #f00 } /* #rgb */
em { color: #ff0000 } /* #rrggbb */
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }
The format of an RGB value in hexadecimal notation is a '#' immediately
followed by either three or six hexadecimal characters. The three-digit
RGB notation (#rgb) is converted into six-digit form (#rrggbb) by
replicating digits, not by adding zeros. For example, #fb0 expands to
#ffbb00. This ensures that white (#ffffff) can be specified with the
short notation (#fff) and removes any dependencies on the color depth
of the display.
The format of an RGB value in the functional notation is 'rgb('
followed by a comma-separated list of three numerical values (either
three integer values or three percentage values) followed by ')'. The
integer value 255 corresponds to 100%, and to F or FF in the
hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF.
White space characters are allowed around the numerical values.
All RGB colors are specified in the sRGB color space (see [SRGB]). User
agents may vary in the fidelity with which they represent these colors,
but using sRGB provides an unambiguous and objectively measurable
definition of what the color should be, which can be related to
international standards (see [COLORIMETRY]).
Conforming user agents may limit their color-displaying efforts to
performing a gamma-correction on them. sRGB specifies a display gamma
of 2.2 under specified viewing conditions. User agents should adjust
the colors given in CSS such that, in combination with an output
device's "natural" display gamma, an effective display gamma of 2.2 is
produced. See the section on gamma correction for further details. Note
that only colors specified in CSS are affected; e.g., images are
expected to carry their own color information.
Values outside the device gamut should be clipped or mapped into the
gamut when the gamut is known: the red, green, and blue values must be
changed to fall within the range supported by the device. Users agents
may perform higher quality mapping of colors from one gamut to another.
For a typical CRT monitor, whose device gamut is the same as sRGB, the
four rules below are equivalent:
Example(s):
em { color: rgb(255,0,0) } /* integer range 0 - 255 */
em { color: rgb(300,0,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) } /* clipped to rgb(100%,0%,0%) */
Other devices, such as printers, have different gamuts than sRGB; some
colors outside the 0..255 sRGB range will be representable (inside the
device gamut), while other colors inside the 0..255 sRGB range will be
outside the device gamut and will thus be mapped.
Note. Mapping or clipping of color values should be done to the actual
device gamut if known (which may be larger or smaller than 0..255).
4.3.7 Strings
Strings can either be written with double quotes or with single quotes.
Double quotes cannot occur inside double quotes, unless escaped (e.g.,
as '\"' or as '\22'). Analogously for single quotes (e.g., "\'" or
"\27").
Example(s):
"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''
A string cannot directly contain a newline. To include a newline in a
string, use an escape representing the line feed character in ISO-10646
(U+000A), such as "\A" or "\00000a". This character represents the
generic notion of "newline" in CSS. See the 'content' property for an
example.
It is possible to break strings over several lines, for esthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). For instance, the following two selectors are
exactly the same:
Example(s):
a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}
4.3.8 Unsupported Values
If a UA does not support a particular value, it should ignore that
value when parsing style sheets, as if that value was an illegal value.
For example:
Example(s):
h3 {
display: inline;
display: run-in;
}
A UA that supports the 'run-in' value for the 'display' property will
accept the first display declaration and then "write over" that value
with the second display declaration. A UA that does not support the
'run-in' value will process the first display declaration and ignore
the second display declaration.
4.4 CSS style sheet representation
A CSS style sheet is a sequence of characters from the Universal
Character Set (see [ISO10646]). For transmission and storage, these
characters must be encoded by a character encoding that supports the
set of characters available in US-ASCII (e.g., UTF-8, ISO 8859-x, SHIFT
JIS, etc.). For a good introduction to character sets and character
encodings, please consult the HTML 4 specification ([HTML4], chapter
5). See also the XML 1.0 specification ([XML10], sections 2.2 and
4.3.3, and Appendix F).
When a style sheet is embedded in another document, such as in the
STYLE element or "style" attribute of HTML, the style sheet shares the
character encoding of the whole document.
When a style sheet resides in a separate file, user agents must observe
the following priorities when determining a style sheet's character
encoding (from highest priority to lowest):
1. An HTTP "charset" parameter in a "Content-Type" field (or similar
parameters in other protocols)
2. BOM and/or @charset (see below)
3. or other metadata from the linking mechanism (if
any)
4. charset of referring style sheet or document (if any)
5. Assume UTF-8
Authors using an @charset rule must place the rule at the very
beginning of the style sheet, preceded by no characters. (If a byte
order mark is appropriate for the encoding used, it may precede the
@charset rule.)
After "@charset", authors specify the name of a character encoding (in
quotes). For example:
@charset "ISO-8859-1";
@charset must be written literally, i.e., the 10 characters '@charset
"' (lowercase, no backslash escapes), followed by the encoding name,
followed by '";'.
The name must be a charset name as described in the IANA registry. See
[CHARSETS] for a complete list of charsets. Authors should use the
charset names marked as "preferred MIME name" in the IANA registry.
User agents must support at least the UTF-8 encoding.
User agents must ignore any @charset rule not at the beginning of the
style sheet. When user agents detect the character encoding using the
BOM and/or the @charset rule, they should follow the following rules:
* Except as specified in these rules, all @charset rules are ignored.
* The encoding is detected based on the stream of bytes that begins
the style sheet. The following table gives a set of possibilities
for initial byte sequences (written in hexadecimal). The first row
that matches the beginning of the style sheet gives the result of
encoding detection based on the BOM and/or @charset rule. If no
rows match, the encoding cannot be detected based on the BOM and/or
@charset rule. The notation (...)* refers to repetition for which
the best match is the one that repeats as few times as possible.
The bytes marked "XX" are those used to determine the name of the
encoding, by treating them, in the order given, as a sequence of
ASCII characters. Bytes marked "YY" are similar, but need to be
transcoded into ASCII as noted. User agents may ignore entries in
the table if they do not support any encodings relevant to the
entry.
Initial Bytes Result
EF BB BF 40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
EF BB BF UTF-8
40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
FE FF 00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00
XX)* 00 22 00 3B as specified (with BE endianness if not specified)
00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)* 00
22 00 3B as specified (with BE endianness if not specified)
FF FE 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX
00)* 22 00 3B 00 as specified (with LE endianness if not specified)
40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)* 22
00 3B 00 as specified (with LE endianness if not specified)
00 00 FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72
00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00
XX)* 00 00 00 22 00 00 00 3B as specified (with BE endianness if not
specified)
00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73
00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00 00 00
22 00 00 00 3B as specified (with BE endianness if not specified)
00 00 FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00
00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX
00)* 00 00 22 00 00 00 3B 00 as specified (with 2143 endianness if not
specified)
00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00
00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00 00 22
00 00 00 3B 00 as specified (with 2143 endianness if not specified)
FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00
00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00
00)* 00 22 00 00 00 3B 00 00 as specified (with 3412 endianness if not
specified)
00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00
00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00 22 00
00 00 3B 00 00 as specified (with 3412 endianness if not specified)
FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00
73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00
00)* 22 00 00 00 3B 00 00 00 as specified (with LE endianness if not
specified)
40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00
65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22 00 00
00 3B 00 00 00 as specified (with LE endianness if not specified)
00 00 FE FF UTF-32-BE
FF FE 00 00 UTF-32-LE
00 00 FF FE UTF-32-2143
FE FF 00 00 UTF-32-3412
FE FF UTF-16-BE
FF FE UTF-16-LE
7C 83 88 81 99 A2 85 A3 40 7F (YY)* 7F 5E as specified, transcoded from
EBCDIC to ASCII
AE 83 88 81 99 A2 85 A3 40 FC (YY)* FC 5E as specified, transcoded from
IBM1026 to ASCII
00 63 68 61 72 73 65 74 20 22 (YY)* 22 3B as specified, transcoded from
GSM 03.38 to ASCII
analogous patterns User agents may support additional, analogous,
patterns if they support encodings that are not handled by the patterns
here
* If the encoding is detected based on one of the entries in the
table above marked "as specified", the user agent ignores the style
sheet if it does not parse an appropriate @charset rule at the
beginning of the stream of characters resulting from decoding in
the chosen @charset. This ensures that:
+ @charset rules should only function if they are in the
encoding of the style sheet,
+ byte order marks are ignored only in encodings that support a
byte order mark, and
+ encoding names cannot contain newlines.
User agents must ignore style sheets in unknown encodings.
4.4.1 Referring to characters not represented in a character encoding
A style sheet may have to refer to characters that cannot be
represented in the current character encoding. These characters must be
written as escaped references to ISO 10646 characters. These escapes
serve the same purpose as numeric character references in HTML or XML
documents (see [HTML4], chapters 5 and 25).
The character escape mechanism should be used when only a few
characters must be represented this way. If most of a style sheet
requires escaping, authors should encode it with a more appropriate
encoding (e.g., if the style sheet contains a lot of Greek characters,
authors might use "ISO-8859-7" or "UTF-8").
Intermediate processors using a different character encoding may
translate these escaped sequences into byte sequences of that encoding.
Intermediate processors must not, on the other hand, alter escape
sequences that cancel the special meaning of an ASCII character.
Conforming user agents must correctly map to ISO-10646 all characters
in any character encodings that they recognize (or they must behave as
if they did).
For example, a style sheet transmitted as ISO-8859-1 (Latin-1) cannot
contain Greek letters directly: "kouro*s" (Greek: "kouros") has to be
written as "\3BA\3BF\3C5\3C1\3BF\3C2".
Note. In HTML 4, numeric character references are interpreted in
"style" attribute values but not in the content of the STYLE element.
Because of this asymmetry, we recommend that authors use the CSS
character escape mechanism rather than numeric character references for
both the "style" attribute and the STYLE element. For example, we
recommend:
...
rather than:
...
__________________________________________________________________
__________________________________________________________________
5 Selectors
Contents
* 5.1 Pattern matching
* 5.2 Selector syntax
+ 5.2.1 Grouping
* 5.3 Universal selector
* 5.4 Type selectors
* 5.5 Descendant selectors
* 5.6 Child selectors
* 5.7 Adjacent sibling selectors
* 5.8 Attribute selectors
+ 5.8.1 Matching attributes and attribute values
+ 5.8.2 Default attribute values in DTDs
+ 5.8.3 Class selectors
* 5.9 ID selectors
* 5.10 Pseudo-elements and pseudo-classes
* 5.11 Pseudo-classes
+ 5.11.1 :first-child pseudo-class
+ 5.11.2 The link pseudo-classes: :link and :visited
+ 5.11.3 The dynamic pseudo-classes: :hover, :active, and :focus
+ 5.11.4 The language pseudo-class: :lang
* 5.12 Pseudo-elements
+ 5.12.1 The :first-line pseudo-element
+ 5.12.2 The :first-letter pseudo-element
+ 5.12.3 The :before and :after pseudo-elements
5.1 Pattern matching
In CSS, pattern matching rules determine which style rules apply to
elements in the document tree. These patterns, called selectors, may
range from simple element names to rich contextual patterns. If all
conditions in the pattern are true for a certain element, the selector
matches the element.
The case-sensitivity of document language element names in selectors
depends on the document language. For example, in HTML, element names
are case-insensitive, but in XML they are case-sensitive.
The following table summarizes CSS 2.1 selector syntax:
Pattern Meaning Described in section
* Matches any element. Universal selector
E Matches any E element (i.e., an element of type E). Type selectors
E F Matches any F element that is a descendant of an E element.
Descendant selectors
E > F Matches any F element that is a child of an element E. Child
selectors
E:first-child Matches element E when E is the first child of its
parent. The :first-child pseudo-class
E:link
E:visited Matches element E if E is the source anchor of a hyperlink of
which the target is not yet visited (:link) or already visited
(:visited). The link pseudo-classes
E:active
E:hover
E:focus Matches E during certain user actions. The dynamic
pseudo-classes
E:lang(c) Matches element of type E if it is in (human) language c (the
document language specifies how language is determined). The :lang()
pseudo-class
E + F Matches any F element immediately preceded by a sibling element
E. Adjacent selectors
E[foo] Matches any E element with the "foo" attribute set (whatever the
value). Attribute selectors
E[foo="warning"] Matches any E element whose "foo" attribute value is
exactly equal to "warning". Attribute selectors
E[foo~="warning"] Matches any E element whose "foo" attribute value is
a list of space-separated values, one of which is exactly equal to
"warning". Attribute selectors
E[lang|="en"] Matches any E element whose "lang" attribute has a
hyphen-separated list of values beginning (from the left) with "en".
Attribute selectors
DIV.warning Language specific. (In HTML, the same as
DIV[class~="warning"].) Class selectors
E#myid Matches any E element with ID equal to "myid". ID selectors
5.2 Selector syntax
A simple selector is either a type selector or universal selector
followed immediately by zero or more attribute selectors, ID selectors,
or pseudo-classes, in any order. The simple selector matches if all of
its components match.
Note: the terminology used here in CSS 2.1 is different from what is
used in CSS3. For example, a "simple selector" refers to a smaller part
of a selector in CSS3 than in CSS 2.1. See the CSS3 Selectors module
[CSS3SEL].
A selector is a chain of one or more simple selectors separated by
combinators. Combinators are: white space, ">", and "+". White space
may appear between a combinator and the simple selectors around it.
The elements of the document tree that match a selector are called
subjects of the selector. A selector consisting of a single simple
selector matches any element satisfying its requirements. Prepending a
simple selector and combinator to a chain imposes additional matching
constraints, so the subjects of a selector are always a subset of the
elements matching the last simple selector.
One pseudo-element may be appended to the last simple selector in a
chain, in which case the style information applies to a subpart of each
subject.
5.2.1 Grouping
When several selectors share the same declarations, they may be grouped
into a comma-separated list.
Example(s):
In this example, we condense three rules with identical declarations
into one. Thus,
h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }
is equivalent to:
h1, h2, h3 { font-family: sans-serif }
CSS offers other "shorthand" mechanisms as well, including multiple
declarations and shorthand properties.
5.3 Universal selector
The universal selector, written "*", matches the name of any element
type. It matches any single element in the document tree.
If the universal selector is not the only component of a simple
selector, the "*" may be omitted. For example:
* *[lang=fr] and [lang=fr] are equivalent.
* *.warning and .warning are equivalent.
* *#myid and #myid are equivalent.
5.4 Type selectors
A type selector matches the name of a document language element type. A
type selector matches every instance of the element type in the
document tree.
Example(s):
The following rule matches all H1 elements in the document tree:
h1 { font-family: sans-serif }
5.5 Descendant selectors
At times, authors may want selectors to match an element that is the
descendant of another element in the document tree (e.g., "Match those
EM elements that are contained by an H1 element"). Descendant selectors
express such a relationship in a pattern. A descendant selector is made
up of two or more selectors separated by white space. A descendant
selector of the form "A B" matches when an element B is an arbitrary
descendant of some ancestor element A.
Example(s):
For example, consider the following rules:
h1 { color: red }
em { color: red }
Although the intention of these rules is to add emphasis to text by
changing its color, the effect will be lost in a case such as:
This headline is very important
We address this case by supplementing the previous rules with a rule
that sets the text color to blue whenever an EM occurs anywhere within
an H1:
h1 { color: red }
em { color: red }
h1 em { color: blue }
The third rule will match the EM in the following fragment:
This headline
is very important
Example(s):
The following selector:
div * p
matches a P element that is a grandchild or later descendant of a DIV
element. Note the white space on either side of the "*" is not part of
the universal selector; the white space is a combinator indicating that
the DIV must be the ancestor of some element, and that that element
must be an ancestor of the P.
Example(s):
The selector in the following rule, which combines descendant and
attribute selectors, matches any element that (1) has the "href"
attribute set and (2) is inside a P that is itself inside a DIV:
div p *[href]
5.6 Child selectors
A child selector matches when an element is the child of some element.
A child selector is made up of two or more selectors separated by ">".
Example(s):
The following rule sets the style of all P elements that are children
of BODY:
body > P { line-height: 1.3 }
Example(s):
The following example combines descendant selectors and child
selectors:
div ol>li p
It matches a P element that is a descendant of an LI; the LI element
must be the child of an OL element; the OL element must be a descendant
of a DIV. Notice that the optional white space around the ">"
combinator has been left out.
For information on selecting the first child of an element, please see
the section on the :first-child pseudo-class below.
5.7 Adjacent sibling selectors
Adjacent sibling selectors have the following syntax: E1 + E2, where E2
is the subject of the selector. The selector matches if E1 and E2 share
the same parent in the document tree and E1 immediately precedes E2,
ignoring non-element nodes (such as text nodes and comments).
Example(s):
Thus, the following rule states that when a P element immediately
follows a MATH element, it should not be indented:
math + p { text-indent: 0 }
The next example reduces the vertical space separating an H1 and an H2
that immediately follows it:
h1 + h2 { margin-top: -5mm }
Example(s):
The following rule is similar to the one in the previous example,
except that it adds a class selector. Thus, special formatting only
occurs when H1 has class="opener":
h1.opener + h2 { margin-top: -5mm }
5.8 Attribute selectors
CSS 2.1 allows authors to specify rules that match elements which have
certain attributes defined in the source document.
5.8.1 Matching attributes and attribute values
Attribute selectors may match in four ways:
[att]
Match when the element sets the "att" attribute, whatever the
value of the attribute.
[att=val]
Match when the element's "att" attribute value is exactly "val".
[att~=val]
Represents an element with the att attribute whose value is a
white space-separated list of words, one of which is exactly
"val". If "val" contains white space, it will never represent
anything (since the words are separated by spaces). If "val" is
the empty string, it will never represent anything either.
[att|=val]
Represents an element with the att attribute, its value either
being exactly "val" or beginning with "val" immediately followed
by "-" (U+002D). This is primarily intended to allow language
subcode matches (e.g., the hreflang attribute on the a element
in HTML) as described in RFC 3066 ([RFC3066]) or its successor.
For lang (or xml:lang) language subcode matching, please see the
:lang pseudo-class.
Attribute values must be identifiers or strings. The case-sensitivity
of attribute names and values in selectors depends on the document
language.
Example(s):
For example, the following attribute selector matches all H1 elements
that specify the "title" attribute, whatever its value:
h1[title] { color: blue; }
Example(s):
In the following example, the selector matches all SPAN elements whose
"class" attribute has exactly the value "example":
span[class=example] { color: blue; }
Multiple attribute selectors can be used to refer to several attributes
of an element, or even several times to the same attribute.
Example(s):
Here, the selector matches all SPAN elements whose "hello" attribute
has exactly the value "Cleveland" and whose "goodbye" attribute has
exactly the value "Columbus":
span[hello="Cleveland"][goodbye="Columbus"] { color: blue; }
Example(s):
The following selectors illustrate the differences between "=" and
"~=". The first selector will match, for example, the value "copyright
copyleft copyeditor" for the "rel" attribute. The second selector will
only match when the "href" attribute has the value
"http://www.w3.org/".
a[rel~="copyright"]
a[href="http://www.w3.org/"]
Example(s):
The following rule hides all elements for which the value of the "lang"
attribute is "fr" (i.e., the language is French).
*[lang=fr] { display : none }
Example(s):
The following rule will match for values of the "lang" attribute that
begin with "en", including "en", "en-US", and "en-cockney":
*[lang|="en"] { color : red }
Example(s):
Similarly, the following aural style sheet rules allow a script to be
read aloud in different voices for each role:
DIALOGUE[character=romeo]
{ voice-family: "Laurence Olivier", charles, male }
DIALOGUE[character=juliet]
{ voice-family: "Vivien Leigh", victoria, female }
5.8.2 Default attribute values in DTDs
Matching takes place on attribute values in the document tree. Default
attribute values may be defined in a DTD or elsewhere, but cannot
always be selected by attribute selectors. Style sheets should be
designed so that they work even if the default values are not included
in the document tree.
More precisely, a UA is not required to read an "external subset" of
the DTD but is required to look for default attribute values in the
document's "internal subset." (See [XML10] for definitions of these
subsets.)
A UA that recognizes an XML namespace [XMLNAMESPACES] is not required
to use its knowledge of that namespace to treat default attribute
values as if they were present in the document. (E.g., an XHTML UA is
not required to use its built-in knowledge of the XHTML DTD.)
Note that, typically, implementations choose to ignore external
subsets.
Example(s):
Example:
For example, consider an element EXAMPLE with an attribute "notation"
that has a default value of "decimal". The DTD fragment might be
If the style sheet contains the rules
EXAMPLE[notation=decimal] { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
the first rule will not match elements whose "notation" attribute is
set by default, i.e. not set explicitly. To catch all cases, the
attribute selector for the default value must be dropped:
EXAMPLE { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
Here, because the selector EXAMPLE[notation=octal] is more specific
than the type selector alone, the style declarations in the second rule
will override those in the first for elements that have a "notation"
attribute value of "octal". Care has to be taken that all property
declarations that are to apply only to the default case are overridden
in the non-default cases' style rules.
5.8.3 Class selectors
Working with HTML, authors may use the period (.) notation as an
alternative to the ~= notation when representing the class attribute.
Thus, for HTML, div.value and div[class~=value] have the same meaning.
The attribute value must immediately follow the "period" (.). UAs may
apply selectors using the period (.) notation in XML documents if the
UA has namespace specific knowledge that allows it to determine which
attribute is the "class" attribute for the respective namespace. One
such example of namespace specific knowledge is the prose in the
specification for a particular namespace (e.g. SVG 1.1 [SVG11]
describes the SVG "class" attribute and how a UA should interpret it,
and similarly MathML 2.0 [MATH20] describes the MathML "class"
attribute.)
Example(s):
For example, we can assign style information to all elements with
class~="pastoral" as follows:
*.pastoral { color: green } /* all elements with class~=pastoral */
or just
.pastoral { color: green } /* all elements with class~=pastoral */
The following assigns style only to H1 elements with class~="pastoral":
H1.pastoral { color: green } /* H1 elements with class~=pastoral */
Given these rules, the first H1 instance below would not have green
text, while the second would:
Not green
Very green
To match a subset of "class" values, each value must be preceded by a
".".
Example(s):
For example, the following rule matches any P element whose "class"
attribute has been assigned a list of space-separated values that
includes "pastoral" and "marine":
p.marine.pastoral { color: green }
This rule matches when class="pastoral blue aqua marine" but does not
match for class="pastoral blue".
Note. CSS gives so much power to the "class" attribute, that authors
could conceivably design their own "document language" based on
elements with almost no associated presentation (such as DIV and SPAN
in HTML) and assigning style information through the "class" attribute.
Authors should avoid this practice since the structural elements of a
document language often have recognized and accepted meanings and
author-defined classes may not.
Note: If an element has multiple class attributes, their values must be
concatenated with spaces between the values before searching for the
class. As of this time the working group is not aware of any manner in
which this situation can be reached, however, so this behavior is
explicitly non-normative in this specification.
5.9 ID selectors
Document languages may contain attributes that are declared to be of
type ID. What makes attributes of type ID special is that no two such
attributes can have the same value; whatever the document language, an
ID attribute can be used to uniquely identify its element. In HTML all
ID attributes are named "id"; XML applications may name ID attributes
differently, but the same restriction applies.
The ID attribute of a document language allows authors to assign an
identifier to one element instance in the document tree. CSS ID
selectors match an element instance based on its identifier. A CSS ID
selector contains a "#" immediately followed by the ID value, which
must be an identifier.
Note that CSS does not specify how a UA knows the ID attribute of an
element. The UA may, e.g., read a document's DTD, have the information
hard-coded or ask the user.
Example(s):
The following ID selector matches the H1 element whose ID attribute has
the value "chapter1":
h1#chapter1 { text-align: center }
In the following example, the style rule matches the element that has
the ID value "z98y". The rule will thus match for the P element:
Match P
Wide text
In the next example, however, the style rule will only match an H1
element that has an ID value of "z98y". The rule will not match the P
element in this example:
Match H1 only
Wide text
ID selectors have a higher specificity than attribute selectors. For
example, in HTML, the selector #p123 is more specific than [id=p123] in
terms of the cascade.
Note. In XML 1.0 [XML10], the information about which attribute
contains an element's IDs is contained in a DTD. When parsing XML, UAs
do not always read the DTD, and thus may not know what the ID of an
element is. If a style sheet designer knows or suspects that this will
be the case, he should use normal attribute selectors instead:
[name=p371] instead of #p371. However, the cascading order of normal
attribute selectors is different from ID selectors. It may be necessary
to add an "!important" priority to the declarations: [name=p371]
{color: red ! important}.
If an element has multiple ID attributes, all of them must be treated
as IDs for that element for the purposes of the ID selector. Such a
situation could be reached using mixtures of xml:id [XMLID], DOM3 Core
[DOM-LEVEL-3-CORE], XML DTDs [XML10] and namespace-specific knowledge.
5.10 Pseudo-elements and pseudo-classes
In CSS 2.1, style is normally attached to an element based on its
position in the document tree. This simple model is sufficient for many
cases, but some common publishing scenarios may not be possible due to
the structure of the document tree. For instance, in HTML 4 (see
[HTML4]), no element refers to the first line of a paragraph, and
therefore no simple CSS selector may refer to it.
CSS introduces the concepts of pseudo-elements and pseudo-classes to
permit formatting based on information that lies outside the document
tree.
* Pseudo-elements create abstractions about the document tree beyond
those specified by the document language. For instance, document
languages do not offer mechanisms to access the first letter or
first line of an element's content. CSS pseudo-elements allow style
sheet designers to refer to this otherwise inaccessible
information. Pseudo-elements may also provide style sheet designers
a way to assign style to content that does not exist in the source
document (e.g., the :before and :after pseudo-elements give access
to generated content).
* Pseudo-classes classify elements on characteristics other than
their name, attributes or content; in principle characteristics
that cannot be deduced from the document tree. Pseudo-classes may
be dynamic, in the sense that an element may acquire or lose a
pseudo-class while a user interacts with the document. The
exceptions are ':first-child', which can be deduced from the
document tree, and ':lang()', which can be deduced from the
document tree in some cases.
Neither pseudo-elements nor pseudo-classes appear in the document
source or document tree.
Pseudo-classes are allowed anywhere in selectors while pseudo-elements
may only be appended after the last simple selector of the selector.
Pseudo-element and pseudo-class names are case-insensitive.
Some pseudo-classes are mutually exclusive, while others can be applied
simultaneously to the same element. In case of conflicting rules, the
normal cascading order determines the outcome.
5.11 Pseudo-classes
5.11.1 :first-child pseudo-class
The :first-child pseudo-class matches an element that is the first
child element of some other element.
Example(s):
In the following example, the selector matches any P element that is
the first child of a DIV element. The rule suppresses indentation for
the first paragraph of a DIV:
div > p:first-child { text-indent: 0 }
This selector would match the P inside the DIV of the following
fragment:
The last P before the note.
The first P inside the note.
but would not match the second P in the following fragment:
The last P before the note.
Note
The first P inside the note.
Example(s):
The following rule sets the font weight to 'bold' for any EM element
that is some descendant of a P element that is a first child:
p:first-child em { font-weight : bold }
Note that since anonymous boxes are not part of the document tree, they
are not counted when calculating the first child.
For example, the EM in:
abc default
is the first child of the P.
The following two selectors are equivalent:
* > a:first-child /* A is first child of any element */
a:first-child /* Same */
5.11.2 The link pseudo-classes: :link and :visited
User agents commonly display unvisited links differently from
previously visited ones. CSS provides the pseudo-classes ':link' and
':visited' to distinguish them:
* The :link pseudo-class applies for links that have not yet been
visited.
* The :visited pseudo-class applies once the link has been visited by
the user.
UAs may return a visited link to the (unvisited) ':link' state at some
point.
The two states are mutually exclusive.
The document language determines which elements are hyperlink source
anchors. For example, in HTML4, the link pseudo-classes apply to A
elements with an "href" attribute. Thus, the following two CSS 2.1
declarations have similar effect:
a:link { color: red }
:link { color: red }
Example(s):
If the following link:
external link
has been visited, this rule:
a.external:visited { color: blue }
will cause it to be blue.
Note. It is possible for style sheet authors to abuse the :link and
:visited pseudo-classes to determine which sites a user has visited
without the user's consent.
UAs may therefore treat all links as unvisited links, or implement
other measures to preserve the user's privacy while rendering visited
and unvisited links differently. See [P3P] for more information about
handling privacy.
5.11.3 The dynamic pseudo-classes: :hover, :active, and :focus
Interactive user agents sometimes change the rendering in response to
user actions. CSS provides three pseudo-classes for common cases:
* The :hover pseudo-class applies while the user designates an
element (with some pointing device), but does not activate it. For
example, a visual user agent could apply this pseudo-class when the
cursor (mouse pointer) hovers over a box generated by the element.
User agents not supporting interactive media do not have to support
this pseudo-class. Some conforming user agents supporting
interactive media may not be able to support this pseudo-class
(e.g., a pen device).
* The :active pseudo-class applies while an element is being
activated by the user. For example, between the times the user
presses the mouse button and releases it.
* The :focus pseudo-class applies while an element has the focus
(accepts keyboard events or other forms of text input).
An element may match several pseudo-classes at the same time.
CSS doesn't define which elements may be in the above states, or how
the states are entered and left. Scripting may change whether elements
react to user events or not, and different devices and UAs may have
different ways of pointing to, or activating elements.
CSS 2.1 doesn't define if the parent of an element that is ':active' or
':hover' is also in that state.
User agents are not required to reflow a currently displayed document
due to pseudo-class transitions. For instance, a style sheet may
specify that the 'font-size' of an :active link should be larger than
that of an inactive link, but since this may cause letters to change
position when the reader selects the link, a UA may ignore the
corresponding style rule.
Example(s):
a:link { color: red } /* unvisited links */
a:visited { color: blue } /* visited links */
a:hover { color: yellow } /* user hovers */
a:active { color: lime } /* active links */
Note that the A:hover must be placed after the A:link and A:visited
rules, since otherwise the cascading rules will hide the 'color'
property of the A:hover rule. Similarly, because A:active is placed
after A:hover, the active color (lime) will apply when the user both
activates and hovers over the A element.
Example(s):
An example of combining dynamic pseudo-classes:
a:focus { background: yellow }
a:focus:hover { background: white }
The last selector matches A elements that are in pseudo-class :focus
and in pseudo-class :hover.
For information about the presentation of focus outlines, please
consult the section on dynamic focus outlines.
Note. In CSS1, the ':active' pseudo-class was mutually exclusive with
':link' and ':visited'. That is no longer the case. An element can be
both ':visited' and ':active' (or ':link' and ':active') and the normal
cascading rules determine which style declarations apply.
Note. Also note that in CSS1, the ':active' pseudo-class only applied
to links.
5.11.4 The language pseudo-class: :lang
If the document language specifies how the human language of an element
is determined, it is possible to write selectors in CSS that match an
element based on its language. For example, in HTML [HTML4], the
language is determined by a combination of the "lang" attribute, the
META element, and possibly by information from the protocol (such as
HTTP headers). XML uses an attribute called xml:lang, and there may be
other document language-specific methods for determining the language.
The pseudo-class ':lang(C)' matches if the element is in language C.
Whether there is a match is based solely on the identifier C being
either equal to, or a hyphen-separated substring of, the element's
language value, in the same way as if performed by the '|=' operator.
The matching of C against the element's language value is performed
case-insensitively. The identifier C doesn't have to be a valid
language name.
C must not be empty.
Note: It is recommended that documents and protocols indicate language
using codes from RFC 3066 [RFC3066] or its successor, and by means of
"xml:lang" attributes in the case of XML-based documents [XML10]. See
"FAQ: Two-letter or three-letter language codes."
Example(s):
The following rules set the quotation marks for an HTML document that
is either in Canadian French or German:
html:lang(fr-ca) { quotes: '« ' ' »' }
html:lang(de) { quotes: '»' '«' '\2039' '\203A' }
:lang(fr) > Q { quotes: '« ' ' »' }
:lang(de) > Q { quotes: '»' '«' '\2039' '\203A' }
The second pair of rules actually set the 'quotes' property on Q
elements according to the language of its parent. This is done because
the choice of quote marks is typically based on the language of the
element around the quote, not the quote itself: like this piece of
French "à l'improviste" in the middle of an English text uses the
English quotation marks.
Note the difference between [lang|=xx] and :lang(xx). In this HTML
example, only the BODY matches [lang|=fr] (because it has a LANG
attribute) but both the BODY and the P match :lang(fr) (because both
are in French).
Je suis Français.
5.12 Pseudo-elements
5.12.1 The :first-line pseudo-element
The :first-line pseudo-element applies special styles to the contents
of the first formatted line of a paragraph. For instance:
p:first-line { text-transform: uppercase }
The above rule means "change the letters of the first line of every
paragraph to uppercase". However, the selector "P:first-line" does not
match any real HTML element. It does match a pseudo-element that
conforming user agents will insert at the beginning of every paragraph.
Note that the length of the first line depends on a number of factors,
including the width of the page, the font size, etc. Thus, an ordinary
HTML paragraph such as:
This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.
the lines of which happen to be broken as follows:
THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.
might be "rewritten" by user agents to include the fictional tag
sequence for :first-line. This fictional tag sequence helps to show how
properties are inherited.
This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.
If a pseudo-element breaks up a real element, the desired effect can
often be described by a fictional tag sequence that closes and then
re-opens the element. Thus, if we mark up the previous paragraph with a
SPAN element:
This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.
the user agent could simulate start and end tags for SPAN when
inserting the fictional tag sequence for :first-line.
This is a
somewhat long HTML
paragraph that will be
broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.
The :first-line pseudo-element can only be attached to a block-level
element, inline-block, table-caption or a table-cell.
The "first formatted line" of an element may occur inside a block-level
descendant in the same flow (i.e., a block-level descendant that is not
positioned and not a float). E.g., the first line of the DIV in
This line...
is the first line of the P (assuming
that both P and DIV are block-level).
The first line of a table-cell or inline-block cannot be the first
formatted line of an ancestor element. Thus, in
Hello Goodbye
etcetera
the first formatted
line of the DIV is not the line "Hello".
Note that the first line of the P in this fragment:
First...
doesn't contain any letters (assuming the default style for BR in HTML
4). The word "First" is not on the first formatted line.
A UA should act as if the fictional start tags of the first-line
pseudo-elements were nested just inside the innermost enclosing
block-level element. (Since CSS1 and CSS2 were silent on this case,
authors should not rely on this behavior.) Here is an example. The
fictional tag sequence for
First paragraph
Second paragraph
is
First paragraph
Second paragraph
The :first-line pseudo-element is similar to an inline-level element,
but with certain restrictions. The following properties apply to a
:first-line pseudo-element: font properties, color property, background
properties, 'word-spacing', 'letter-spacing', 'text-decoration',
'vertical-align', 'text-transform', 'line-height',. UAs may apply other
properties as well.
5.12.2 The :first-letter pseudo-element
The :first-letter pseudo-element must select the first letter of the
first line of a block, if it is not preceded by any other content (such
as images or inline tables) on its line. The :first-letter
pseudo-element may be used for "initial caps" and "drop caps", which
are common typographical effects. This type of initial letter is
similar to an inline-level element if its 'float' property is 'none',
otherwise it is similar to a floated element.
These are the properties that apply to :first-letter pseudo-elements:
font properties, 'text-decoration', 'text-transform', 'letter-spacing',
'word-spacing' (when appropriate), 'line-height', 'float',
'vertical-align' (only if 'float' is 'none'), margin properties,
padding properties, border properties, color property, background
properties. UAs may apply other properties as well. To allow UAs to
render a typographically correct drop cap or initial cap, the UA may
choose a line-height, width and height based on the shape of the
letter, unlike for normal elements. CSS3 is expected to have specific
properties that apply to first-letter.
This example shows a possible rendering of an initial cap. Note that
the 'line-height' that is inherited by the first-letter pseudo-element
is 1.1, but the UA in this example has computed the height of the first
letter differently, so that it doesn't cause any unnecessary space
between the first two lines. Also note that the fictional start tag of
the first letter is inside the SPAN, and thus the font weight of the
first letter is normal, not bold as the SPAN:
p { line-height: 1.1 }
p:first-letter { font-size: 3em; font-weight: normal }
span { font-weight: bold }
...
Het hemelsche gerecht heeft zich ten lange lesten
Erbarremt over my en mijn benaeuwde vesten
En arme burgery, en op mijn volcx gebed
En dagelix geschrey de bange stad ontzet.
Image illustrating the :first-letter pseudo-element
The following CSS 2.1 will make a drop cap initial letter span about
two lines:
Drop cap initial letter
The first few words of an article
in The Economist.
This example might be formatted as follows:
Image illustrating the combined effect of the :first-letter and
:first-line pseudo-elements [D]
The fictional tag sequence is:
T
he first
few words of an article in the Economist.
Note that the :first-letter pseudo-element tags abut the content (i.e.,
the initial character), while the :first-line pseudo-element start tag
is inserted right after the start tag of the block element.
In order to achieve traditional drop caps formatting, user agents may
approximate font sizes, for example to align baselines. Also, the glyph
outline may be taken into account when formatting.
Punctuation (i.e, characters defined in Unicode [UNICODE] in the "open"
(Ps), "close" (Pe), "initial" (Pi). "final" (Pf) and "other" (Po)
punctuation classes), that precedes or follows the first letter should
be included, as in:
Quotes that precede the first letter should be included. [D]
The ':first-letter' also applies if the first letter is in fact a
digit, e.g., the "6" in "67 million dollars is a lot of money."
The :first-letter pseudo-element applies to block, list-item,
table-cell, table-caption and inline-block elements.
The :first-letter pseudo-element can be used with all such elements
that contain text, or that have a descendant in the same flow that
contains text. A UA should act as if the fictional start tag of the
first-letter pseudo-element is just before the first text of the
element, even if that first text is in a descendant.
Example(s):
Here is an example. The fictional tag sequence for this HTML fragment:
The first text.
is:
The first text.
The first letter of a table-cell or inline-block cannot be the first
letter of an ancestor element. Thus, in
Hello Goodbye
etcetera
the first letter of
the DIV is not the letter "H". In fact, the DIV doesn't have a first
letter.
The first letter must occur on the first formatted line. For example,
in this fragment:
First... the first line doesn't contain any
letters and ':first-letter' doesn't match anything (assuming the
default style for BR in HTML 4). In particular, it does not match the
"F" of "First."
If an element is a list item ('display: list-item'), the
':first-letter' applies to the first letter in the principal box after
the marker. UAs may ignore ':first-letter' on list items with
'list-style-position: inside'. If an element has ':before' or ':after'
content, the ':first-letter applies to the first letter of the element
including that content.
E.g., after the rule 'p:before {content: "Note: "}', the selector
'p:first-letter' matches the "N" of "Note".
Some languages may have specific rules about how to treat certain
letter combinations. In Dutch, for example, if the letter combination
"ij" appears at the beginning of a word, both letters should be
considered within the :first-letter pseudo-element.
If the letters that would form the first-letter are not in the same
element, such as "'T" in
'T..., the UA may create a first-letter
pseudo-element from one of the elements, both elements, or simply not
create a pseudo-element.
Similarly, if the first letter(s) of the block are not at the start of
the line (for example due to bidirectional reordering), then the UA
need not create the pseudo-element(s).
Example(s):
The following example illustrates how overlapping pseudo-elements may
interact. The first letter of each P element will be green with a font
size of '24pt'. The rest of the first formatted line will be 'blue'
while the rest of the paragraph will be 'red'.
p { color: red; font-size: 12pt }
p:first-letter { color: green; font-size: 200% }
p:first-line { color: blue }
Some text that ends up on two lines
Assuming that a line break will occur before the word "ends", the
fictional tag sequence for this fragment might be:
S
ome text that
ends up on two lines
Note that the :first-letter element is inside the :first-line element.
Properties set on :first-line are inherited by :first-letter, but are
overridden if the same property is set on :first-letter.
5.12.3 The :before and :after pseudo-elements
The ':before' and ':after' pseudo-elements can be used to insert
generated content before or after an element's content. They are
explained in the section on generated text.
Example(s):
h1:before {content: counter(chapno, upper-roman) ". "}
When the :first-letter and :first-line pseudo-elements are applied to
an element having content generated using :before and :after, they
apply to the first letter or line of the element including the
generated content.
Example(s):
p.special:before {content: "Special! "}
p.special:first-letter {color: #ffd800}
This will render the "S" of "Special!" in gold.
__________________________________________________________________
__________________________________________________________________
6 Assigning property values, Cascading, and Inheritance
Contents
* 6.1 Specified, computed, and actual values
+ 6.1.1 Specified values
+ 6.1.2 Computed values
+ 6.1.3 Used values
+ 6.1.4 Actual values
* 6.2 Inheritance
+ 6.2.1 The 'inherit' value
* 6.3 The @import rule
* 6.4 The cascade
+ 6.4.1 Cascading order
+ 6.4.2 !important rules
+ 6.4.3 Calculating a selector's specificity
+ 6.4.4 Precedence of non-CSS presentational hints
6.1 Specified, computed, and actual values
Once a user agent has parsed a document and constructed a document
tree, it must assign, for every element in the tree, a value to every
property that applies to the target media type.
The final value of a property is the result of a four-step calculation:
the value is determined through specification (the "specified value"),
then resolved into a value that is used for inheritance (the "computed
value"), then converted into an absolute value if necessary (the "used
value"), and finally transformed according to the limitations of the
local environment (the "actual value").
6.1.1 Specified values
User agents must first assign a specified value to each property based
on the following mechanisms (in order of precedence):
1. If the cascade results in a value, use it.
2. Otherwise, if the property is inherited and the element is not the
root of the document tree, use the computed value of the parent
element.
3. Otherwise use the property's initial value. The initial value of
each property is indicated in the property's definition.
6.1.2 Computed values
Specified values are resolved to computed values during the cascade;
for example URIs are made absolute and 'em' and 'ex' units are computed
to pixel or absolute lengths. Computing a value never requires the user
agent to render the document.
The computed value of URIs that the UA cannot resolve to absolute URIs
is the specified value.
When the specified value is not 'inherit', the computed value of a
property is determined as specified by the Computed Value line in the
definition of the property. See the section on inheritance for the
definition of computed values when the specified value is 'inherit'.
The computed value exists even when the property doesn't apply, as
defined by the 'Applies To' line. However, some properties may define
the computed value of a property for an element to depend on whether
the property applies to that element.
6.1.3 Used values
Computed values are processed as far as possible without formatting the
document. Some values, however, can only be determined when the
document is being laid out. For example, if the width of an element is
set to be a certain percentage of its containing block, the width
cannot be determined until the width of the containing block has been
determined. The used value is the result of taking the computed value
and resolving any remaining dependencies into an absolute value.
6.1.4 Actual values
A used value is in principle the value used for rendering, but a user
agent may not be able to make use of the value in a given environment.
For example, a user agent may only be able to render borders with
integer pixel widths and may therefore have to approximate the computed
width, or the user agent may be forced to use only black and white
shades instead of full colour. The actual value is the used value after
any approximations have been applied.
6.2 Inheritance
Some values are inherited by the children of an element in the document
tree, as described above. Each property defines whether it is inherited
or not.
Suppose there is an H1 element with an emphasizing element (EM) inside:
The headline is important!
If no color has been assigned to the EM element, the emphasized "is"
will inherit the color of the parent element, so if H1 has the color
blue, the EM element will likewise be in blue.
When inheritance occurs, elements inherit computed values. The computed
value from the parent element becomes both the specified value and the
computed value on the child.
Example(s):
For example, given the following style sheet:
body { font-size: 10pt }
h1 { font-size: 130% }
and this document fragment:
A large heading
the 'font-size' property for the H1 element will have the computed
value '13pt' (130% times 10pt, the parent's value). Since the computed
value of 'font-size' is inherited, the EM element will have the
computed value '13pt' as well. If the user agent does not have the 13pt
font available, the actual value of 'font-size' for both H1 and EM
might be, for example, '12pt'.
6.2.1 The 'inherit' value
Each property may also have a specified value of 'inherit', which means
that, for a given element, the property takes the same computed value
as the property for the element's parent. The 'inherit' value can be
used to strengthen inherited values, and it can also be used on
properties that are not normally inherited.
If the 'inherit' value is set on the root element, the property is
assigned its initial value.
Example(s):
In the example below, the 'color' and 'background' properties are set
on the BODY element. On all other elements, the 'color' value will be
inherited and the background will be transparent. If these rules are
part of the user's style sheet, black text on a white background will
be enforced throughout the document.
body {
color: black !important;
background: white !important;
}
* {
color: inherit !important;
background: transparent !important;
}
6.3 The @import rule
The '@import' rule allows users to import style rules from other style
sheets. In CSS 2.1, any @import rules must precede all other rules
(except the @charset rule, if present). See the section on parsing for
when user agents must ignore @import rules. The '@import' keyword must
be followed by the URI of the style sheet to include. A string is also
allowed; it will be interpreted as if it had url(...) around it.
Example(s):
The following lines are equivalent in meaning and illustrate both
'@import' syntaxes (one with "url()" and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");
So that user agents can avoid retrieving resources for unsupported
media types, authors may specify media-dependent @import rules. These
conditional imports specify comma-separated media types after the URI.
Example(s):
The following rules illustrate how @import rules can be made
media-dependent:
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
In the absence of any media types, the import is unconditional.
Specifying 'all' for the medium has the same effect. The import only
takes effect if the target medium matches the media list.
A target medium matches a media list if one of the items in the media
list is the target medium or 'all'.
Note that Media Queries [MEDIAQ] extends the syntax of media lists and
the definition of matching.
When the same style sheet is imported or linked to a document in
multiple places, user agents must process (or act as though they do)
each link as though the link were to a separate style sheet.
6.4 The cascade
Style sheets may have three different origins: author, user, and user
agent.
* Author. The author specifies style sheets for a source document
according to the conventions of the document language. For
instance, in HTML, style sheets may be included in the document or
linked externally.
* User: The user may be able to specify style information for a
particular document. For example, the user may specify a file that
contains a style sheet or the user agent may provide an interface
that generates a user style sheet (or behaves as if it did).
* User agent: Conforming user agents must apply a default style sheet
(or behave as if they did). A user agent's default style sheet
should present the elements of the document language in ways that
satisfy general presentation expectations for the document language
(e.g., for visual browsers, the EM element in HTML is presented
using an italic font). See A sample style sheet for HTML for a
recommended default style sheet for HTML documents.
Note that the user may modify system settings (e.g. system colors)
that affect the default style sheet. However, some user agent
implementations make it impossible to change the values in the
default style sheet.
Style sheets from these three origins will overlap in scope, and they
interact according to the cascade.
The CSS cascade assigns a weight to each style rule. When several rules
apply, the one with the greatest weight takes precedence.
By default, rules in author style sheets have more weight than rules in
user style sheets. Precedence is reversed, however, for "!important"
rules. All user and author rules have more weight than rules in the
UA's default style sheet.
6.4.1 Cascading order
To find the value for an element/property combination, user agents must
apply the following sorting order:
1. Find all declarations that apply to the element and property in
question, for the target media type. Declarations apply if the
associated selector matches the element in question and the target
medium matches the media list on all @media rules containing the
declaration and on all links on the path through which the style
sheet was reached.
2. Sort according to importance (normal or important) and origin
(author, user, or user agent). In ascending order of precedence:
1. user agent declarations
2. user normal declarations
3. author normal declarations
4. author important declarations
5. user important declarations
3. Sort rules with the same importance and origin by specificity of
selector: more specific selectors will override more general ones.
Pseudo-elements and pseudo-classes are counted as normal elements
and classes, respectively.
4. Finally, sort by order specified: if two declarations have the same
weight, origin and specificity, the latter specified wins.
Declarations in imported style sheets are considered to be before
any declarations in the style sheet itself.
Apart from the "!important" setting on individual declarations, this
strategy gives author's style sheets higher weight than those of the
reader. User agents must give the user the ability to turn off the
influence of specific author style sheets, e.g., through a pull-down
menu. Conformance to UAAG 1.0 checkpoint 4.14 satisfies this condition
[UAAG10].
6.4.2 !important rules
CSS attempts to create a balance of power between author and user style
sheets. By default, rules in an author's style sheet override those in
a user's style sheet (see cascade rule 3).
However, for balance, an "!important" declaration (the delimiter token
"!" and keyword "important" follow the declaration) takes precedence
over a normal declaration. Both author and user style sheets may
contain "!important" declarations, and user "!important" rules override
author "!important" rules. This CSS feature improves accessibility of
documents by giving users with special requirements (large fonts, color
combinations, etc.) control over presentation.
Declaring a shorthand property (e.g., 'background') to be "!important"
is equivalent to declaring all of its sub-properties to be
"!important".
Example(s):
The first rule in the user's style sheet in the following example
contains an "!important" declaration, which overrides the corresponding
declaration in the author's style sheet. The second declaration will
also win due to being marked "!important". However, the third rule in
the user's style sheet is not "!important" and will therefore lose to
the second rule in the author's style sheet (which happens to set style
on a shorthand property). Also, the third author rule will lose to the
second author rule since the second rule is "!important". This shows
that "!important" declarations have a function also within author style
sheets.
/* From the user's style sheet */
p { text-indent: 1em ! important }
p { font-style: italic ! important }
p { font-size: 18pt }
/* From the author's style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }
6.4.3 Calculating a selector's specificity
A selector's specificity is calculated as follows:
* count 1 if the declaration is from is a 'style' attribute rather
than a rule with a selector, 0 otherwise (= a) (In HTML, values of
an element's "style" attribute are style sheet rules. These rules
have no selectors, so a=1, b=0, c=0, and d=0.)
* count the number of ID attributes in the selector (= b)
* count the number of other attributes and pseudo-classes in the
selector (= c)
* count the number of element names and pseudo-elements in the
selector (= d)
The specificity is based only on the form of the selector. In
particular, a selector of the form "[id=p33]" is counted as an
attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is
defined as an "ID" in the source document's DTD.
Concatenating the four numbers a-b-c-d (in a number system with a large
base) gives the specificity.
Example(s):
Some examples:
* {} /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */
li {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */
li:first-line {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul li {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul ol+li {} /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */
h1 + *[rel=up]{} /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */
ul ol li.red {} /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */
li.red.level {} /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */
#x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
style="" /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */
In the above example, the color of the P element would be green. The
declaration in the "style" attribute will override the one in the STYLE
element because of cascading rule 3, since it has a higher specificity.
6.4.4 Precedence of non-CSS presentational hints
The UA may choose to honor presentational attributes in an HTML source
document. If so, these attributes are translated to the corresponding
CSS rules with specificity equal to 0, and are treated as if they were
inserted at the start of the author style sheet. They may therefore be
overridden by subsequent style sheet rules. In a transition phase, this
policy will make it easier for stylistic attributes to coexist with
style sheets.
For HTML, any attribute that is not in the following list should be
considered presentational: abbr, accept-charset, accept, accesskey,
action, alt, archive, axis, charset, checked, cite, class, classid,
code, codebase, codetype, colspan, coords, data, datetime, declare,
defer, dir, disabled, enctype, for, headers, href, hreflang,
http-equiv, id, ismap, label, lang, language, longdesc, maxlength,
media, method, multiple, name, nohref, object, onblur, onchange,
onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onload,
onload, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup,
onreset, onselect, onsubmit, onunload, onunload, profile, prompt,
readonly, rel, rev, rowspan, scheme, scope, selected, shape, span, src,
standby, start, style, summary, title, type (except on LI, OL and UL
elements), usemap, value, valuetype, version.
For other languages, all document language-based styling should be
handled in the user agent style sheet.
Example(s):
The following user style sheet would override the font weight of 'b'
elements in all documents, and the color of 'font' elements with color
attributes in XML documents. It would not affect the color of any
'font' elements with color attributes in HTML documents:
b { font-weight: normal; }
font[color] { color: orange; }
The following, however, would override the color of font elements in
all documents:
font[color] { color: orange ! important; }
__________________________________________________________________
__________________________________________________________________
7 Media types
Contents
* 7.1 Introduction to media types
* 7.2 Specifying media-dependent style sheets
+ 7.2.1 The @media rule
* 7.3 Recognized media types
+ 7.3.1 Media groups
7.1 Introduction to media types
One of the most important features of style sheets is that they specify
how a document is to be presented on different media: on the screen, on
paper, with a speech synthesizer, with a braille device, etc.
Certain CSS properties are only designed for certain media (e.g., the
'page-break-before' property only applies to paged media). On occasion,
however, style sheets for different media types may share a property,
but require different values for that property. For example, the
'font-size' property is useful both for screen and print media. The two
media types are different enough to require different values for the
common property; a document will typically need a larger font on a
computer screen than on paper. Therefore, it is necessary to express
that a style sheet, or a section of a style sheet, applies to certain
media types.
7.2 Specifying media-dependent style sheets
There are currently two ways to specify media dependencies for style
sheets:
* Specify the target medium from a style sheet with the @media or
@import at-rules.
Example(s):
@import url("fancyfonts.css") screen;
@media print {
/* style sheet for print goes here */
}
* Specify the target medium within the document language. For
example, in HTML 4 ([HTML4]), the "media" attribute on the LINK
element specifies the target media of an external style sheet:
Link to a target medium
The body...
The @import rule is defined in the chapter on the cascade.
7.2.1 The @media rule
An @media rule specifies the target media types (separated by commas)
of a set of statements (delimited by curly braces). Invalid statements
must be ignored per 4.1.7 "Rule sets, declaration blocks, and
selectors" and 4.2 "Rules for handling parsing errors." The @media
construct allows style sheet rules for various media in the same style
sheet:
@media print {
body { font-size: 10pt }
}
@media screen {
body { font-size: 13px }
}
@media screen, print {
body { line-height: 1.2 }
}
Style rules outside of @media rules apply to all media types that the
style sheet applies to. At-rules inside @media are invalid in CSS2.1.
7.3 Recognized media types
The names chosen for CSS media types reflect target devices for which
the relevant properties make sense. In the following list of CSS media
types the names of media types are normative, but the descriptions are
informative. Likewise, the "Media" field in the description of each
property is informative.
all
Suitable for all devices.
braille
Intended for braille tactile feedback devices.
embossed
Intended for paged braille printers.
handheld
Intended for handheld devices (typically small screen, limited
bandwidth).
print
Intended for paged material and for documents viewed on screen
in print preview mode. Please consult the section on paged media
for information about formatting issues that are specific to
paged media.
projection
Intended for projected presentations, for example projectors.
Please consult the section on paged media for information about
formatting issues that are specific to paged media.
screen
Intended primarily for color computer screens.
speech
Intended for speech synthesizers. Note: CSS2 had a similar media
type called 'aural' for this purpose. See the appendix on aural
style sheets for details.
tty
Intended for media using a fixed-pitch character grid (such as
teletypes, terminals, or portable devices with limited display
capabilities). Authors should not use pixel units with the "tty"
media type.
tv
Intended for television-type devices (low resolution, color,
limited-scrollability screens, sound available).
Media type names are case-insensitive.
Media types are mutually exclusive in the sense that a user agent can
only support one media type when rendering a document. However, user
agents may use different media types on different canvases. For
example, a document may (simultaneously) be shown in 'screen' mode on
one canvas and 'print' mode on another canvas.
Note that a multimodal media type is still only one media type. The
'tv' media type, for example, is a multimodal media type that renders
both visually and aurally to a single canvas.
@media and @import rules with unknown media types are treated as if the
unknown media types are not present.
Example(s):
For example, in the following snippet, the rule on the P element
applies in 'screen' mode (even though the '3D' media type is not
known).
@media screen, 3D {
P { color: green; }
}
Note. Future updates of CSS may extend the list of media types. Authors
should not rely on media type names that are not yet defined by a CSS
specification.
7.3.1 Media groups
This section is informative, not normative.
Each CSS property definition specifies which media types the property
applies to. Since properties generally apply to several media types,
the "Applies to media" section of each property definition lists media
groups rather than individual media types. Each property applies to all
media types in the media groups listed in its definition.
CSS 2.1 defines the following media groups:
* continuous or paged.
* visual, audio, speech, or tactile.
* grid (for character grid devices), or bitmap.
* interactive (for devices that allow user interaction), or static
(for those that don't).
* all (includes all media types)
The following table shows the relationships between media groups and
media types:
CAPTION: Relationship between media groups and media types
Media Types Media Groups
continuous/paged visual/audio/speech/tactile grid/bitmap
interactive/static
braille continuous tactile grid both
embossed paged tactile grid static
handheld both visual, audio, speech both both
print paged visual bitmap static
projection paged visual bitmap interactive
screen continuous visual, audio bitmap both
speech continuous speech N/A both
tty continuous visual grid both
tv both visual, audio bitmap both
__________________________________________________________________
__________________________________________________________________
8 Box model
Contents
* 8.1 Box dimensions
* 8.2 Example of margins, padding, and borders
* 8.3 Margin properties: 'margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin'
+ 8.3.1 Collapsing margins
* 8.4 Padding properties: 'padding-top', 'padding-right',
'padding-bottom', 'padding-left', and 'padding'
* 8.5 Border properties
+ 8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and 'border-width'
+ 8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and 'border-color'
+ 8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and 'border-style'
+ 8.5.4 Border shorthand properties: 'border-top',
'border-right', 'border-bottom', 'border-left', and 'border'
* 8.6 The box model for inline elements in bidirection context
The CSS box model describes the rectangular boxes that are generated
for elements in the document tree and laid out according to the visual
formatting model.
8.1 Box dimensions
Each box has a content area (e.g., text, an image, etc.) and optional
surrounding padding, border, and margin areas; the size of each area is
specified by properties defined below. The following diagram shows how
these areas relate and the terminology used to refer to pieces of
margin, border, and padding:
Image illustrating the relationship between content, padding, borders,
and margins. [D]
The margin, border, and padding can be broken down into top, right,
bottom, and left segments (e.g., in the diagram, "LM" for left margin,
"RP" for right padding, "TB" for top border, etc.).
The perimeter of each of the four areas (content, padding, border, and
margin) is called an "edge", so each box has four edges:
content edge or inner edge
The content edge surrounds the rectangle given by the width and
height of the box, which often depend on the element's rendered
content. The four content edges define the box's content box.
padding edge
The padding edge surrounds the box padding. If the padding has 0
width, the padding edge is the same as the content edge. The
four padding edges define the box's padding box.
border edge
The border edge surrounds the box's border. If the border has 0
width, the border edge is the same as the padding edge. The four
border edges define the box's border box.
margin edge or outer edge
The margin edge surrounds the box margin. If the margin has 0
width, the margin edge is the same as the border edge. The four
margin edges define the box's margin box.
Each edge may be broken down into a top, right, bottom, and left edge.
The dimensions of the content area of a box -- the content width and
content height -- depend on several factors: whether the element
generating the box has the 'width' or 'height' property set, whether
the box contains text or other boxes, whether the box is a table, etc.
Box widths and heights are discussed in the chapter on visual
formatting model details.
The background style of the content, padding, and border areas of a box
is specified by the 'background' property of the generating element.
Margin backgrounds are always transparent.
8.2 Example of margins, padding, and borders
This example illustrates how margins, padding, and borders interact.
The example HTML document:
Examples of margins, padding, and borders
First element of list
Second element of list is
a bit longer to illustrate wrapping.
results in a document tree with (among other relationships) a UL
element that has two LI children.
The first of the following diagrams illustrates what this example would
produce. The second illustrates the relationship between the margins,
padding, and borders of the UL elements and those of its children LI
elements. (Image is not to scale.)
Image illustrating how parent and child margins, borders, and padding
relate. [D]
Note that:
* The content width for each LI box is calculated top-down; the
containing block for each LI box is established by the UL element.
* The margin box height of each LI box depends on its content height,
plus top and bottom padding, borders, and margins. Note that
vertical margins between the LI boxes collapse.
* The right padding of the LI boxes has been set to zero width (the
'padding' property). The effect is apparent in the second
illustration.
* The margins of the LI boxes are transparent -- margins are always
transparent -- so the background color (yellow) of the UL padding
and content areas shines through them.
* The second LI element specifies a dashed border (the 'border-style'
property).
8.3 Margin properties: 'margin-top', 'margin-right', 'margin-bottom',
'margin-left', and 'margin'
Margin properties specify the width of the margin area of a box. The
'margin' shorthand property sets the margin for all four sides while
the other margin properties only set their respective side. These
properties apply to all elements, but vertical margins will not have
any effect on non-replaced inline elements.
The properties defined in this section refer to the
value type, which may take one of the following values:
Specifies a fixed width.
The percentage is calculated with respect to the width of the
generated box's containing block. Note that this is true for
'margin-top' and 'margin-bottom' as well. If the containing
block's width depends on this element, then the resulting layout
is undefined in CSS 2.1.
auto
See the section on calculating widths and margins for behavior.
Negative values for margin properties are allowed, but there may be
implementation-specific limits.
'margin-top', 'margin-bottom'
Value: | inherit
Initial: 0
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties have no effect on non-replaced inline elements.
'margin-right', 'margin-left'
Value: | inherit
Initial: 0
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties set the top, right, bottom, and left margin of a box.
Example(s):
h1 { margin-top: 2em }
'margin'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements except elements with table display types
other than table-caption, table and inline-table
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'margin' property is a shorthand property for setting 'margin-top',
'margin-right', 'margin-bottom', and 'margin-left' at the same place in
the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom margins are set to the first value and the
right and left margins are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
Example(s):
body { margin: 2em } /* all margins set to 2em */
body { margin: 1em 2em } /* top & bottom = 1em, right & left = 2em */
body { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */
The last rule of the example above is equivalent to the example below:
body {
margin-top: 1em;
margin-right: 2em;
margin-bottom: 3em;
margin-left: 2em; /* copied from opposite side (right) */
}
8.3.1 Collapsing margins
In this specification, the expression collapsing margins means that
adjoining margins (no non-empty content, padding or border areas or
clearance separate them) of two or more boxes (which may be next to one
another or nested) combine to form a single margin.
In CSS 2.1, horizontal margins never collapse.
Vertical margins may collapse between certain boxes:
* Two or more adjoining vertical margins of block boxes in the normal
flow collapse. The resulting margin width is the maximum of the
adjoining margin widths. In the case of negative margins, the
maximum of the absolute values of the negative adjoining margins is
deducted from the maximum of the positive adjoining margins. If
there are no positive margins, the absolute maximum of the negative
adjoining margins is deducted from zero. Note. Adjoining boxes may
be generated by elements that are not related as siblings or
ancestors.
* Vertical margins between a floated box and any other box do not
collapse (not even between a float and its in-flow children).
* Vertical margins of elements that establish new block formatting
contexts (such as floats and elements with 'overflow' other than
'visible') do not collapse with their in-flow children.
* Margins of absolutely positioned boxes do not collapse (not even
with their in-flow children).
* Margins of inline-block elements do not collapse (not even with
their in-flow children).
* If the top and bottom margins of a box are adjoining, then it is
possible for margins to collapse through it. In this case, the
position of the element depends on its relationship with the other
elements whose margins are being collapsed.
+ If the element's margins are collapsed with its parent's top
margin, the top border edge of the box is defined to be the
same as the parent's.
+ Otherwise, either the element's parent is not taking part in
the margin collapsing, or only the parent's bottom margin is
involved. The position of the element's top border edge is the
same as it would have been if the element had a non-zero
bottom border.
An element that has had clearance applied to it never collapses its
top margin with its parent block's bottom margin.
Note that the positions of elements that have been collapsed
through have no effect on the positions of the other elements with
whose margins they are being collapsed; the top border edge
position is only required for laying out descendants of these
elements.
* Margins of the root element's box do not collapse.
The bottom margin of an in-flow block-level element is always adjoining
to the top margin of its next in-flow block-level sibling, unless that
sibling has clearance.
The top margin of an in-flow block-level element is adjoining to its
first in-flow block-level child's top margin if the element has no top
border, no top padding, and the child has no clearance.
The bottom margin of an in-flow block-level element with a 'height' of
'auto' is adjoining to its last in-flow block-level child's bottom
margin if the element has no bottom padding or border.
An element's own margins are adjoining if the 'min-height' property is
zero, and it has neither top or bottom borders nor top or bottom
padding, and it has a 'height' of either 0 or 'auto', and it does not
contain a line box, and all of its in-flow children's margins (if any)
are adjoining.
When an element's own margins collapse, and that element has had
clearance applied to it, its top margin collapses with the adjoining
margins of subsequent siblings but that resulting margin does not
collapse with the bottom margin of the parent block.
Collapsing is based on the used value of 'padding', 'margin', and
'border' (i.e., after resolving any percentages). The collapsed margin
is calculated over the used value of the various margins.
Please consult the examples of margin, padding, and borders for an
illustration of collapsed margins.
8.4 Padding properties: 'padding-top', 'padding-right', 'padding-bottom',
'padding-left', and 'padding'
The padding properties specify the width of the padding area of a box.
The 'padding' shorthand property sets the padding for all four sides
while the other padding properties only set their respective side.
The properties defined in this section refer to the
value type, which may take one of the following values:
Specifies a fixed width.
The percentage is calculated with respect to the width of the
generated box's containing block, even for 'padding-top' and
'padding-bottom'. If the containing block's width depends on
this element, then the resulting layout is undefined in CSS 2.1.
Unlike margin properties, values for padding values cannot be negative.
Like margin properties, percentage values for padding properties refer
to the width of the generated box's containing block.
'padding-top', 'padding-right', 'padding-bottom', 'padding-left'
Value: | inherit
Initial: 0
Applies to: all elements except table-row-group, table-header-group,
table-footer-group, table-row, table-column-group and table-column
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
These properties set the top, right, bottom, and left padding of a box.
Example(s):
blockquote { padding-top: 0.3em }
'padding'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements except table-row-group, table-header-group,
table-footer-group, table-row, table-column-group and table-column
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'padding' property is a shorthand property for setting
'padding-top', 'padding-right', 'padding-bottom', and 'padding-left' at
the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom paddings are set to the first value and the
right and left paddings are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
The surface color or image of the padding area is specified via the
'background' property:
Example(s):
h1 {
background: white;
padding: 1em 2em;
}
The example above specifies a '1em' vertical padding ('padding-top' and
'padding-bottom') and a '2em' horizontal padding ('padding-right' and
'padding-left'). The 'em' unit is relative to the element's font size:
'1em' is equal to the size of the font in use.
8.5 Border properties
The border properties specify the width, color, and style of the border
area of a box. These properties apply to all elements.
Note. Notably for HTML, user agents may render borders for certain user
interface elements (e.g., buttons, menus, etc.) differently than for
"ordinary" elements.
8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and 'border-width'
The border width properties specify the width of the border area. The
properties defined in this section refer to the value
type, which may take one of the following values:
thin
A thin border.
medium
A medium border.
thick
A thick border.
The border's thickness has an explicit value. Explicit border
widths cannot be negative.
The interpretation of the first three values depends on the user agent.
The following relationships must hold, however:
'thin' <='medium' <= 'thick'.
Furthermore, these widths must be constant throughout a document.
'border-top-width', 'border-right-width', 'border-bottom-width',
'border-left-width'
Value: | inherit
Initial: medium
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute length; '0' if the border style is 'none' or
'hidden'
These properties set the width of the top, right, bottom, and left
border of a box.
'border-width'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This property is a shorthand property for setting 'border-top-width',
'border-right-width', 'border-bottom-width', and 'border-left-width' at
the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom borders are set to the first value and the
right and left are set to the second. If there are three values, the
top is set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are four values,
they apply to the top, right, bottom, and left, respectively.
Example(s):
In the examples below, the comments indicate the resulting widths of
the top, right, bottom, and left borders:
h1 { border-width: thin } /* thin thin thin thin */
h1 { border-width: thin thick } /* thin thick thin thick */
h1 { border-width: thin thick medium } /* thin thick medium thick */
8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and 'border-color'
The border color properties specify the color of a box's border.
'border-top-color', 'border-right-color', 'border-bottom-color',
'border-left-color'
Value: | transparent | inherit
Initial: the value of the 'color' property
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: when taken from the 'color' property, the computed
value of 'color'; otherwise, as specified
'border-color'
Value: [ | transparent ]{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-color' property sets the color of the four borders. Values
have the following meanings:
Specifies a color value.
transparent
The border is transparent (though it may have width).
The 'border-color' property can have from one to four values, and the
values are set on the different sides as for 'border-width'.
If an element's border color is not specified with a border property,
user agents must use the value of the element's 'color' property as the
computed value for the border color.
Example(s):
In this example, the border will be a solid black line.
p {
color: black;
background: white;
border: solid;
}
8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and 'border-style'
The border style properties specify the line style of a box's border
(solid, double, dashed, etc.). The properties defined in this section
refer to the value type, which may take one of the
following values:
none
No border; the computed border width is zero.
hidden
Same as 'none', except in terms of border conflict resolution
for table elements.
dotted
The border is a series of dots.
dashed
The border is a series of short line segments.
solid
The border is a single line segment.
double
The border is two solid lines. The sum of the two lines and the
space between them equals the value of 'border-width'.
groove
The border looks as though it were carved into the canvas.
ridge
The opposite of 'groove': the border looks as though it were
coming out of the canvas.
inset
The border makes the box look as though it were embedded in the
canvas.
outset
The opposite of 'inset': the border makes the box look as though
it were coming out of the canvas.
All borders are drawn on top of the box's background. The color of
borders drawn for values of 'groove', 'ridge', 'inset', and 'outset'
depends on the element's border color properties, but UAs may choose
their own algorithm to calculate the actual colors used. For instance,
if the 'border-color' has the value 'silver', then a UA could use a
gradient of colors from white to dark gray to indicate a sloping
border.
'border-top-style', 'border-right-style', 'border-bottom-style',
'border-left-style'
Value: | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
'border-style'
Value: {1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-style' property sets the style of the four borders. It can
have from one to four values, and the values are set on the different
sides as for 'border-width' above.
Example(s):
#xy34 { border-style: solid dotted }
In the above example, the horizontal borders will be 'solid' and the
vertical borders will be 'dotted'.
Since the initial value of the border styles is 'none', no borders will
be visible unless the border style is set.
8.5.4 Border shorthand properties: 'border-top', 'border-right',
'border-bottom', 'border-left', and 'border'
'border-top', 'border-right', 'border-bottom', 'border-left'
Value: [ || || <'border-top-color'> ] |
inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This is a shorthand property for setting the width, style, and color of
the top, right, bottom, and left border of a box.
Example(s):
h1 { border-bottom: thick solid red }
The above rule will set the width, style, and color of the border below
the H1 element. Omitted values are set to their initial values. Since
the following rule does not specify a border color, the border will
have the color specified by the 'color' property:
H1 { border-bottom: thick solid }
'border'
Value: [ || || <'border-top-color'> ] |
inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border' property is a shorthand property for setting the same
width, color, and style for all four borders of a box. Unlike the
shorthand 'margin' and 'padding' properties, the 'border' property
cannot set different values on the four borders. To do so, one or more
of the other border properties must be used.
Example(s):
For example, the first rule below is equivalent to the set of four
rules shown after it:
p { border: solid red }
p {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red
}
Since, to some extent, the properties have overlapping functionality,
the order in which the rules are specified is important.
Example(s):
Consider this example:
blockquote {
border: solid red;
border-left: double;
color: black;
}
In the above example, the color of the left border is black, while the
other borders are red. This is due to 'border-left' setting the width,
style, and color. Since the color value is not given by the
'border-left' property, it will be taken from the 'color' property. The
fact that the 'color' property is set after the 'border-left' property
is not relevant.
8.6 The box model for inline elements in bidirection context
For each line box, UAs must take the inline boxes generated for each
element and render the margins, borders and padding in visual order
(not logical order).
When the element's 'direction' property is 'ltr', the left-most
generated box of the first line box in which the element appears has
the left margin, left border and left padding, and the right-most
generated box of the last line box in which the element appears has the
right padding, right border and right margin.
When the element's 'direction' property is 'rtl', the right-most
generated box of the first line box in which the element appears has
the right padding, right border and right margin, and the left-most
generated box of the last line box in which the element appears has the
left margin, left border and left padding.
__________________________________________________________________
__________________________________________________________________
9 Visual formatting model
Contents
* 9.1 Introduction to the visual formatting model
+ 9.1.1 The viewport
+ 9.1.2 Containing blocks
* 9.2 Controlling box generation
+ 9.2.1 Block-level elements and block boxes
o 9.2.1.1 Anonymous block boxes
+ 9.2.2 Inline-level elements and inline boxes
o 9.2.2.1 Anonymous inline boxes
+ 9.2.3 Run-in boxes
+ 9.2.4 The 'display' property
* 9.3 Positioning schemes
+ 9.3.1 Choosing a positioning scheme: 'position' property
+ 9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
* 9.4 Normal flow
+ 9.4.1 Block formatting contexts
+ 9.4.2 Inline formatting context
+ 9.4.3 Relative positioning
* 9.5 Floats
+ 9.5.1 Positioning the float: the 'float' property
+ 9.5.2 Controlling flow next to floats: the 'clear' property
* 9.6 Absolute positioning
+ 9.6.1 Fixed positioning
* 9.7 Relationships between 'display', 'position', and 'float'
* 9.8 Comparison of normal flow, floats, and absolute positioning
+ 9.8.1 Normal flow
+ 9.8.2 Relative positioning
+ 9.8.3 Floating a box
+ 9.8.4 Absolute positioning
* 9.9 Layered presentation
+ 9.9.1 Specifying the stack level: the 'z-index' property
* 9.10 Text direction: the 'direction' and 'unicode-bidi' properties
9.1 Introduction to the visual formatting model
This chapter and the next describe the visual formatting model: how
user agents process the document tree for visual media.
In the visual formatting model, each element in the document tree
generates zero or more boxes according to the box model. The layout of
these boxes is governed by:
* box dimensions and type.
* positioning scheme (normal flow, float, and absolute positioning).
* relationships between elements in the document tree.
* external information (e.g., viewport size, intrinsic dimensions of
images, etc.).
The properties defined in this chapter and the next apply to both
continuous media and paged media. However, the meanings of the margin
properties vary when applied to paged media (see the page model for
details).
The visual formatting model does not specify all aspects of formatting
(e.g., it does not specify a letter-spacing algorithm). Conforming user
agents may behave differently for those formatting issues not covered
by this specification.
9.1.1 The viewport
User agents for continuous media generally offer users a viewport (a
window or other viewing area on the screen) through which users consult
a document. User agents may change the document's layout when the
viewport is resized (see the initial containing block).
When the viewport is smaller than the area of the canvas on which the
document is rendered, the user agent should offer a scrolling
mechanism. There is at most one viewport per canvas, but user agents
may render to more than one canvas (i.e., provide different views of
the same document).
9.1.2 Containing blocks
In CSS 2.1, many box positions and sizes are calculated with respect to
the edges of a rectangular box called a containing block. In general,
generated boxes act as containing blocks for descendant boxes; we say
that a box "establishes" the containing block for its descendants. The
phrase "a box's containing block" means "the containing block in which
the box lives," not the one it generates.
Each box is given a position with respect to its containing block, but
it is not confined by this containing block; it may overflow.
The details of how a containing block's dimensions are calculated are
described in the next chapter.
9.2 Controlling box generation
The following sections describe the types of boxes that may be
generated in CSS 2.1. A box's type affects, in part, its behavior in
the visual formatting model. The 'display' property, described below,
specifies a box's type.
9.2.1 Block-level elements and block boxes
Block-level elements are those elements of the source document that are
formatted visually as blocks (e.g., paragraphs). Several values of the
'display' property make an element block-level: 'block', 'list-item',
and 'run-in' (part of the time; see run-in boxes), and 'table'.
Block-level elements (except for display 'table' elements, which are
described in a later chapter) generate a principal block box that
contains either only block boxes or only inline boxes. The principal
block box establishes the containing block for descendant boxes and
generated content and is also the box involved in any positioning
scheme. Principal block boxes participate in a block formatting
context.
Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements. These additional boxes are placed
with respect to the principal box.
9.2.1.1 Anonymous block boxes
In a document like this:
Some text
More text
(and assuming the DIV and the P both have 'display: block'), the DIV
appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an anonymous
block box around "Some text".
diagram showing the three boxes for the example above [D]
Diagram showing the three boxes, of which one is anonymous, for the
example above.
In other words: if a block box (such as that generated for the DIV
above) has another block box or run-in box inside it (such as the P
above), then we force it to have only block boxes and run-in boxes
inside it.
When an inline box contains a block box, the inline box (and its inline
ancestors within the same line box) are broken around the block. The
line boxes before the break and after the break are enclosed in
anonymous boxes, and the block box becomes a sibling of those anonymous
boxes. When such an inline box is affected by relative positioning, the
relative positioning also affects the block box.
Example(s):
This model would apply in the following example if the following rules:
body { display: inline }
p { display: block }
were used with this HTML document:
Anonymous text interrupted by a block
This is anonymous text before the P.
This is the content of P.
This is anonymous text after the P.
The BODY element contains a chunk (C1) of anonymous text followed by a
block-level element followed by another chunk (C2) of anonymous text.
The resulting boxes would be an anonymous block box around the BODY,
containing an anonymous block box around C1, the P block box, and
another anonymous block box around C2.
The properties of anonymous boxes are inherited from the enclosing
non-anonymous box (e.g. in the example just below the subsection
heading "Anonymous block boxes", the one for DIV). Non-inherited
properties have their initial value. For example, the font of the
anonymous box is inherited from the DIV, but the margins will be 0.
Properties set on elements that cause anonymous block boxes to be
generated still apply to the boxes and content of that element. For
example, if a border had been set on the BODY element in the above
example, the border would be drawn around C1 (open at the end of the
line) and C2 (open at the start of the line).
Some user agents have implemented borders on inlines containing blocks
in other ways, e.g. by wrapping such nested blocks inside "anonymous
line boxes" and thus drawing inline borders around such boxes. As CSS1
and CSS2 did not define this behavior, CSS1-only and CSS2-only user
agents may implement this alternative model and still claim conformance
to this part of CSS 2.1. This does not apply to UAs developed after
this specification was released.
9.2.2 Inline-level elements and inline boxes
Inline-level elements are those elements of the source document that do
not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text within a paragraph, inline images,
etc.). Several values of the 'display' property make an element inline:
'inline', 'inline-table', 'inline-block' and 'run-in' (part of the
time; see run-in boxes). Inline-level elements generate inline boxes.
9.2.2.1 Anonymous inline boxes
In a document with HTML markup like this:
Some emphasized text
The
generates a block box, with several inline boxes inside it. The
box for "emphasized" is an inline box generated by an inline element
(), but the other boxes ("Some" and "text") are inline boxes
generated by a block-level element (
). The latter are called
anonymous inline boxes, because they don't have an associated
inline-level element.
Such anonymous inline boxes inherit inheritable properties from their
block parent box. Non-inherited properties have their initial value. In
the example, the color of the anonymous inline boxes is inherited from
the P, but the background is transparent.
White space content that would subsequently be collapsed away according
to the 'white-space' property does not generate any anonymous inline
boxes.
If it is clear from the context which type of anonymous box is meant,
both anonymous inline boxes and anonymous block boxes are simply called
anonymous boxes in this specification.
There are more types of anonymous boxes that arise when formatting
tables.
9.2.3 Run-in boxes
A run-in box behaves as follows:
1. If the run-in box contains a block box, the run-in box becomes a
block box.
2. If a sibling block box (that does not float and is not absolutely
positioned) follows the run-in box, the run-in box becomes the
first inline box of the block box. A run-in cannot run in to a
block that already starts with a run-in or that itself is a run-in.
3. Otherwise, the run-in box becomes a block box.
A 'run-in' box is useful for run-in headers, as in this example:
A run-in box example
A run-in heading.
And a paragraph of text that
follows it.
This example might be formatted as:
A run-in heading. And a
paragraph of text that
follows it.
Despite appearing visually part of the following block box, a run-in
element still inherits properties from its parent in the source tree.
Please consult the section on generated content for information about
how run-in boxes interact with generated content.
9.2.4 The 'display' property
'display'
Value: inline | block | list-item | run-in | inline-block | table |
inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none | inherit
Initial: inline
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: see text
The values of this property have the following meanings:
block
This value causes an element to generate a block box.
inline-block
This value causes an element to generate a block box, which
itself is flowed as a single inline box, similar to a replaced
element. The inside of an inline-block is formatted as a block
box, and the element itself is formatted as an inline replaced
element.
inline
This value causes an element to generate one or more inline
boxes.
list-item
This value causes an element (e.g., LI in HTML) to generate a
principal block box and a list-item inline box. For information
about lists and examples of list formatting, please consult the
section on lists.
none
This value causes an element to not appear in the formatting
structure (i.e., in visual media the element generates no boxes
and has no effect on layout). Descendant elements do not
generate any boxes either; the element and its content are
removed from the formatting structure entirely. This behavior
cannot be overridden by setting the 'display' property on the
descendants.
Please note that a display of 'none' does not create an
invisible box; it creates no box at all. CSS includes mechanisms
that enable an element to generate boxes in the formatting
structure that affect formatting but are not visible themselves.
Please consult the section on visibility for details.
run-in
This value creates either block or inline boxes, depending on
context. Properties apply to run-in boxes based on their final
status (inline-level or block-level).
table, inline-table, table-row-group, table-column, table-column-group,
table-header-group, table-footer-group, table-row, table-cell,
and table-caption
These values cause an element to behave like a table element
(subject to restrictions described in the chapter on tables).
The computed value is the same as the specified value, except for
positioned and floating elements (see Relationships between 'display',
'position', and 'float') and for the root element. For the root
element, the computed value is changed as described in the section on
the relationships between 'display', 'position', and 'float'.
Note that although the initial value of 'display' is 'inline', rules in
the user agent's default style sheet may override this value. See the
sample style sheet for HTML 4 in the appendix.
Example(s):
Here are some examples of the 'display' property:
p { display: block }
em { display: inline }
li { display: list-item }
img { display: none } /* Don't display images */
9.3 Positioning schemes
In CSS 2.1, a box may be laid out according to three positioning
schemes:
1. Normal flow. In CSS 2.1, normal flow includes block formatting of
block boxes, inline formatting of inline boxes, relative
positioning of block or inline boxes, and positioning of run-in
boxes.
2. Floats. In the float model, a box is first laid out according to
the normal flow, then taken out of the flow and shifted to the left
or right as far as possible. Content may flow along the side of a
float.
3. Absolute positioning. In the absolute positioning model, a box is
removed from the normal flow entirely (it has no impact on later
siblings) and assigned a position with respect to a containing
block.
Note. CSS 2.1's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks (e.g.,
invisible images) used for layout effects.
9.3.1 Choosing a positioning scheme: 'position' property
The 'position' and 'float' properties determine which of the CSS 2.1
positioning algorithms is used to calculate the position of a box.
'position'
Value: static | relative | absolute | fixed | inherit
Initial: static
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The values of this property have the following meanings:
static
The box is a normal box, laid out according to the normal flow.
The 'top', 'right', 'bottom', and 'left' properties do not
apply.
relative
The box's position is calculated according to the normal flow
(this is called the position in normal flow). Then the box is
offset relative to its normal position. When a box B is
relatively positioned, the position of the following box is
calculated as though B were not offset. The effect of
'position:relative' on table-row-group, table-header-group,
table-footer-group, table-row, table-column-group, table-column,
table-cell, and table-caption elements is undefined.
absolute
The box's position (and possibly size) is specified with the
'top', 'right', 'bottom', and 'left' properties. These
properties specify offsets with respect to the box's containing
block. Absolutely positioned boxes are taken out of the normal
flow. This means they have no impact on the layout of later
siblings. Also, though absolutely positioned boxes have margins,
they do not collapse with any other margins.
fixed
The box's position is calculated according to the 'absolute'
model, but in addition, the box is fixed with respect to some
reference. As with the 'absolute' model, the box's margins do
not collapse with any other margins. In the case of handheld,
projection, screen, tty, and tv media types, the box is fixed
with respect to the viewport and doesn't move when scrolled. In
the case of the print media type, the box is rendered on every
page, and is fixed with respect to the page box, even if the
page is seen through a viewport (in the case of a print-preview,
for example). For other media types, the presentation is
undefined. Authors may wish to specify 'fixed' in a
media-dependent way. For instance, an author may want a box to
remain at the top of the viewport on the screen, but not at the
top of each printed page. The two specifications may be
separated by using an @media rule, as in:
Example(s):
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
UAs must not paginate the content of fixed boxes. Note that UAs
may print invisible content in other ways. See "Content outside
the page box" in chapter 13.
User agents may treat position as 'static' on the root element.
9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
An element is said to be positioned if its 'position' property has a
value other than 'static'. Positioned elements generate positioned
boxes, laid out according to four properties:
'top'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
This property specifies how far an absolutely positioned box's top
margin edge is offset below the top edge of the box's containing block.
For relatively positioned boxes, the offset is with respect to the top
edges of the box itself (i.e., the box is given a position in the
normal flow, then offset from that position according to these
properties).
'right'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's right margin edge is offset
to the left of the right edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the right
edge of the box itself.
'bottom'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's bottom margin edge is offset
above the bottom of the box's containing block. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself.
'left'
Value: | | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: for 'position:relative', see section Relative
Positioning. For 'position:static', 'auto'. Otherwise: if specified as
a length, the corresponding absolute length; if specified as a
percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's left margin edge is offset to
the right of the left edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the left
edge of the box itself.
The values for the four properties have the following meanings:
The offset is a fixed distance from the reference edge. Negative
values are allowed.
The offset is a percentage of the containing block's width (for
'left' or 'right') or height (for 'top' and 'bottom'). Negative
values are allowed.
auto
For non-replaced elements, the effect of this value depends on
which of related properties have the value 'auto' as well. See
the sections on the width and height of absolutely positioned,
non-replaced elements for details. For replaced elements, the
effect of this value depends only on the intrinsic dimensions of
the replaced content. See the sections on the width and height
of absolutely positioned, replaced elements for details.
9.4 Normal flow
Boxes in the normal flow belong to a formatting context, which may be
block or inline, but not both simultaneously. Block boxes participate
in a block formatting context. Inline boxes participate in an inline
formatting context.
9.4.1 Block formatting contexts
Floats, absolutely positioned elements, inline-blocks, table-cells,
table-captions, and elements with 'overflow' other than 'visible'
(except when that value has been propagated to the viewport) establish
new block formatting contexts.
In a block formatting context, boxes are laid out one after the other,
vertically, beginning at the top of a containing block. The vertical
distance between two sibling boxes is determined by the 'margin'
properties. Vertical margins between adjacent block boxes in a block
formatting context collapse.
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch). This is true even in the presence of floats (although a
box's line boxes may shrink due to the floats), unless the box
establishes a new block formatting context (in which case the box
itself may become narrower due to the floats).
For information about page breaks in paged media, please consult the
section on allowed page breaks.
9.4.2 Inline formatting context
In an inline formatting context, boxes are laid out horizontally, one
after the other, beginning at the top of a containing block. Horizontal
margins, borders, and padding are respected between these boxes. The
boxes may be aligned vertically in different ways: their bottoms or
tops may be aligned, or the baselines of text within them may be
aligned. The rectangular area that contains the boxes that form a line
is called a line box.
The width of a line box is determined by a containing block and the
presence of floats. The height of a line box is determined by the rules
given in the section on line height calculations.
A line box is always tall enough for all of the boxes it contains.
However, it may be taller than the tallest box it contains (if, for
example, boxes are aligned so that baselines line up). When the height
of a box B is less than the height of the line box containing it, the
vertical alignment of B within the line box is determined by the
'vertical-align' property. When several inline boxes cannot fit
horizontally within a single line box, they are distributed among two
or more vertically-stacked line boxes. Thus, a paragraph is a vertical
stack of line boxes. Line boxes are stacked with no vertical separation
and they never overlap.
In general, the left edge of a line box touches the left edge of its
containing block and the right edge touches the right edge of its
containing block. However, floating boxes may come between the
containing block edge and the line box edge. Thus, although line boxes
in the same inline formatting context generally have the same width
(that of the containing block), they may vary in width if available
horizontal space is reduced due to floats. Line boxes in the same
inline formatting context generally vary in height (e.g., one line
might contain a tall image while the others contain only text).
When the total width of the inline boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the 'text-align' property. If that
property has the value 'justify', the user agent may stretch spaces and
words in inline boxes (except for inline-table and inline-block boxes)
as well.
When an inline box exceeds the width of a line box, it is split into
several boxes and these boxes are distributed across several line
boxes. If an inline box cannot be split (e.g. if the inline box
contains a single character, or language specific word breaking rules
disallow a break within the inline box, or if the inline box is
affected by a white-space value of nowrap or pre), then the inline box
overflows the line box.
When an inline box is split, margins, borders, and padding have no
visual effect where the split occurs (or at any split, when there are
several).
Inline boxes may also be split into several boxes within the same line
box due to bidirectional text processing.
Line boxes that contain no text, no preserved white space, no inline
elements with non-zero margins, padding, or borders, and no other
in-flow content (such as images, inline blocks or inline tables), and
don't end with a line feed must be treated as zero-height line boxes.
For the purposes of margin collapsing, this line box must be ignored.
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:
Several emphasized words appear
in this sentence, dear.
The P element generates a block box that contains five inline boxes,
three of which are anonymous:
* Anonymous: "Several"
* EM: "emphasized words"
* Anonymous: "appear"
* STRONG: "in this"
* Anonymous: "sentence, dear."
To format the paragraph, the user agent flows the five boxes into line
boxes. In this example, the box generated for the P element establishes
the containing block for the line boxes. If the containing block is
sufficiently wide, all the inline boxes will fit into a single line
box:
Several emphasized words appear in this sentence, dear.
If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
Several emphasized words appear
in this sentence, dear.
or like this:
Several emphasized
words appear in this
sentence, dear.
In the previous example, the EM box was split into two EM boxes (call
them "split1" and "split2"). Margins, borders, padding, or text
decorations have no visible effect after split1 or before split2.
Consider the following example:
Example of inline flow on several lines
Several emphasized words appear here.
Depending on the width of the P, the boxes may be distributed as
follows:
Image illustrating the effect of line breaking on the display of
margins, borders, and padding. [D]
* The margin is inserted before "emphasized" and after "words".
* The padding is inserted before, above, and below "emphasized" and
after, above, and below "words". A dashed border is rendered on
three sides in each case.
9.4.3 Relative positioning
Once a box has been laid out according to the normal flow or floated,
it may be shifted relative to this position. This is called relative
positioning. Offsetting a box (B1) in this way has no effect on the box
(B2) that follows: B2 is given a position as if B1 were not offset and
B2 is not re-positioned after B1's offset is applied. This implies that
relative positioning may cause boxes to overlap. However, if relative
positioning causes an 'overflow:auto' or 'overflow:scroll' box to have
overflow, the UA must allow the user to access this content (at its
offset position), which, through the creation of scrollbars, may affect
layout.
A relatively positioned box keeps its normal flow size, including line
breaks and the space originally reserved for it. The section on
containing blocks explains when a relatively positioned box establishes
a new containing block.
For relatively positioned elements, 'left' and 'right' move the box(es)
horizontally, without changing their size. 'left' moves the boxes to
the right, and 'right' moves them to the left. Since boxes are not
split or stretched as a result of 'left' or 'right', the computed
values are always: left = -right.
If both 'left' and 'right' are 'auto' (their initial values), the
computed values are '0' (i.e., the boxes stay in their original
position).
If 'left' is 'auto', its computed value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
If 'right' is specified as 'auto', its computed value is minus the
value of 'left'.
If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the 'direction'
property of the containing block is 'ltr, the value of 'left' wins and
'right' becomes -'left'. If 'direction' of the containing block is
'rtl', 'right' wins and 'left' is ignored.
Example(s):
Example. The following three rules are equivalent:
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
The 'top' and 'bottom' properties move relatively positioned element(s)
up or down without changing their size. 'top' moves the boxes down, and
'bottom' moves them up. Since boxes are not split or stretched as a
result of 'top' or 'bottom', the computed values are always: top =
-bottom. If both are 'auto', their computed values are both '0'. If one
of them is 'auto', it becomes the negative of the other. If neither is
'auto', 'bottom' is ignored (i.e., the computed value of 'bottom' will
be minus the value of 'top').
Note. Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the 'visibility'
property). Although relative positioning may be used as a form of
superscripting and subscripting, the line height is not automatically
adjusted to take the positioning into consideration. See the
description of line height calculations for more information.
Examples of relative positioning are provided in the section comparing
normal flow, floats, and absolute positioning.
9.5 Floats
A float is a box that is shifted to the left or right on the current
line. The most interesting characteristic of a float (or "floated" or
"floating" box) is that content may flow along its side (or be
prohibited from doing so by the 'clear' property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact rules governing float behavior
are given in the description of the 'float' property.
A floated box is shifted to the left or right until its outer edge
touches the containing block edge or the outer edge of another float.
If there's a line box, the top of the floated box is aligned with the
top of the current line box.
If there isn't enough horizontal room for the float, it is shifted
downward until either it fits or there are no more floats present.
Since a float is not in the flow, non-positioned block boxes created
before and after the float box flow vertically as if the float didn't
exist. However, line boxes created next to the float are shortened to
make room for the margin box of the float. If a shortened line box is
too small to contain any further content, then it is shifted downward
until either it fits or there are no more floats present. Any content
in the current line before a floated box is reflowed in the first
available line on the other side of the float. In other words, if
inline boxes are placed on the line before a left float is encountered
that fits in the remaining line box space, the left float is placed on
that line, aligned with the top of the line box, and then the inline
boxes already on the line are moved accordingly to the right of the
float (the right being the other side of the left float) and vice versa
for rtl and right floats.
The border box of a table, a block-level replaced element, or an
element in the normal flow that establishes a new block formatting
context (such as an element with 'overflow' other than 'visible') must
not overlap any floats in the same block formatting context as the
element itself. If necessary, implementations should clear the said
element by placing it below any preceding floats, but may place it
adjacent to such floats if there is sufficient space. They may even
make the border box of said element narrower than defined by
section 10.3.3. CSS2 does not define when a UA may put said element
next to the float or by how much said element may become narrower.
Example(s):
Example. In the following document fragment, the containing block is
too narrow to contain the content next to the float, so the content
gets moved to below the floats where it is aligned in the line box
according to the text-align property.
p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }
...
Supercalifragilisticexpialidocious
This fragment might look like this:
Image illustrating the effect of an unbreakable piece of content being
reflowed to just after a float which left insufficient room next to it
for the content to fit.
Several floats may be adjacent, and this model also applies to adjacent
floats in the same line.
Example(s):
The following rule floats all IMG boxes with class="icon" to the left
(and sets the left margin to '0'):
img.icon {
float: left;
margin-left: 0;
}
Consider the following HTML source and style sheet:
Float example
Some sample text that has no other...
The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float. This
document might be formatted as:
Image illustrating how floating boxes interact with margins. [D]
Formatting would have been exactly the same if the document had been:
Some sample text
that has no other...
because the content to the left of the float is displaced by the float
and reflowed down its right side.
As stated in section 8.3.1, the margins of floating boxes never
collapse with margins of adjacent boxes. Thus, in the previous example,
vertical margins do not collapse between the P box and the floated IMG
box.
The contents of floats are stacked as if floats generated new stacking
contexts, except that any positioned elements and elements that
actually create new stacking contexts take part in the float's parent
stacking context. A float can overlap other boxes in the normal flow
(e.g., when a normal flow box next to a float has negative margins).
When this happens, floats are rendered in front of non-positioned
in-flow blocks, but behind in-flow inlines.
Example(s):
Here is another illustration, showing what happens when a float
overlaps borders of elements in the normal flow.
Image showing a floating image that overlaps the borders of two
paragraphs: the borders are interrupted by the image. [D]
A floating image obscures borders of block boxes it overlaps.
The following example illustrates the use of the 'clear' property to
prevent content from flowing next to a float.
Example(s):
Assuming a rule such as this:
p { clear: left }
formatting might look like this:
Image showing a floating image and the effect of 'clear: left' on the
two paragraphs. [D]
Both paragraphs have set 'clear: left', which causes the second
paragraph to be "pushed down" to a position below the float --
"clearance" is added above its top margin to accomplish this (see the
'clear' property).
9.5.1 Positioning the float: the 'float' property
'float'
Value: left | right | none | inherit
Initial: none
Applies to: all, but see 9.7
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether a box should float to the left, right,
or not at all. It may be set for any element, but only applies to
elements that generate boxes that are not absolutely positioned. The
values of this property have the following meanings:
left
The element generates a block box that is floated to the left.
Content flows on the right side of the box, starting at the top
(subject to the 'clear' property).
right
Similar to 'left', except the box is floated to the right, and
content flows on the left side of the box, starting at the top.
none
The box is not floated.
User agents may treat float as 'none' on the root element.
Here are the precise rules that govern the behavior of floats:
1. The left outer edge of a left-floating box may not be to the left
of the left edge of its containing block. An analogous rule holds
for right-floating elements.
2. If the current box is left-floating, and there are any
left-floating boxes generated by elements earlier in the source
document, then for each such earlier box, either the left outer
edge of the current box must be to the right of the right outer
edge of the earlier box, or its top must be lower than the bottom
of the earlier box. Analogous rules hold for right-floating boxes.
3. The right outer edge of a left-floating box may not be to the right
of the left outer edge of any right-floating box that is to the
right of it. Analogous rules hold for right-floating elements.
4. A floating box's outer top may not be higher than the top of its
containing block. When the float occurs between two collapsing
margins, the float is positioned as if it had an otherwise empty
anonymous block parent taking part in the flow. The position of
such a parent is defined by the rules in the section on margin
collapsing.
5. The outer top of a floating box may not be higher than the outer
top of any block or floated box generated by an element earlier in
the source document.
6. The outer top of an element's floating box may not be higher than
the top of any line-box containing a box generated by an element
earlier in the source document.
7. A left-floating box that has another left-floating box to its left
may not have its right outer edge to the right of its containing
block's right edge. (Loosely: a left float may not stick out at the
right edge, unless it is already as far to the left as possible.)
An analogous rule holds for right-floating elements.
8. A floating box must be placed as high as possible.
9. A left-floating box must be put as far to the left as possible, a
right-floating box as far to the right as possible. A higher
position is preferred over one that is further to the left/right.
References to other elements in these rules refer only to other
elements in the same block formatting context as the float.
Example(s):
This HTML fragment results in the b floating to the right.
ab
If the P element's width is enough, the a and the b will be side by
side. It might look like this:
An a at the left side of a box and a b at the right side
9.5.2 Controlling flow next to floats: the 'clear' property
'clear'
Value: none | left | right | both | inherit
Initial: none
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property indicates which sides of an element's box(es) may not be
adjacent to an earlier floating box. The 'clear' property does not
consider floats inside the element itself or in other block formatting
contexts.
For run-in boxes, this property applies to the final block box to which
the run-in box belongs.
Clearance is introduced as spacing above the margin-top of an element.
It is used to push the element vertically (typically downward), past
the float.
Values have the following meanings when applied to non-floating block
boxes:
left
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any left-floating boxes that resulted from elements
earlier in the source document.
right
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any right-floating boxes that resulted from elements
earlier in the source document.
both
The clearance of the generated box is set to the amount
necessary to place the top border edge below the bottom outer
edge of any right-floating and left-floating boxes that resulted
from elements earlier in the source document.
none
No constraint on the box's position with respect to floats.
Computing the clearance of an element on which 'clear' is set is done
by first determining the hypothetical position of the element's top
border edge within its parent block. This position is determined after
the top margin of the element has been collapsed with previous adjacent
margins (including the top margin of the parent block).
If this hypothetical position of the element's top border edge is not
past the relevant floats, then its clearance must be set to the greater
of:
1. The amount necessary to place the border edge of the block even
with the bottom outer edge of the lowest float that is to be
cleared.
2. The amount necessary to make the sum of the following equal to the
distance to which these margins collapsed when the hypothetical
position was calculated:
+ the margins collapsing above the clearance
+ the clearance itself
+ if the block's own margins collapse together: the block's top
margin
+ if the block's own margins do not collapse together: the
margins collapsing below the clearance
Note: The clearance can be negative.
Example(s):
An example of negative clearance is this situation, in which the
clearance is -1em. (Assume none of the elements have borders or
padding):
First paragraph.
Floating paragraph.
Last paragraph.
Explanation: Without the 'clear', the first and last paragraphs'
margins would collapse and the last paragraph's top border edge would
be flush with the top of the floating paragraph. But the 'clear'
requires the top border edge to be below the float, i.e., 2em lower.
That means that the margins must not collapse and clearance must be
added such that clearance + margin-top = 2em, i.e., clearance = 2em -
margin-top = 2em - 3em = -1em.
When the property is set on floating elements, it results in a
modification of the rules for positioning the float. An extra
constraint (#10) is added:
* The top outer edge of the float must be below the bottom outer edge
of all earlier left-floating boxes (in the case of 'clear: left'),
or all earlier right-floating boxes (in the case of 'clear:
right'), or both ('clear: both').
Note. This property applied to all elements in CSS1. Implementations
may therefore have supported this property on all elements. In CSS2 and
CSS 2.1 the 'clear' property only applies to block-level elements.
Therefore authors should only use this property on block-level
elements. If an implementation does support clear on inline elements,
rather than setting a clearance as explained above, the implementation
should force a break and effectively insert one or more empty line
boxes (or shifting the new line box downward as described in section
9.5) to move the top of the cleared inline's line box to below the
respective floating box(es).
9.6 Absolute positioning
In the absolute positioning model, a box is explicitly offset with
respect to its containing block. It is removed from the normal flow
entirely (it has no impact on later siblings). An absolutely positioned
box establishes a new containing block for normal flow children and
absolutely (but not fixed) positioned descendants. However, the
contents of an absolutely positioned element do not flow around any
other boxes. They may obscure the contents of another box (or be
obscured themselves), depending on the stack levels of the overlapping
boxes.
References in this specification to an absolutely positioned element
(or its box) imply that the element's 'position' property has the value
'absolute' or 'fixed'.
9.6.1 Fixed positioning
Fixed positioning is a subcategory of absolute positioning. The only
difference is that for a fixed positioned box, the containing block is
established by the viewport. For continuous media, fixed boxes do not
move when the document is scrolled. In this respect, they are similar
to fixed background images. For paged media, boxes with fixed positions
are repeated on every page. This is useful for placing, for instance, a
signature at the bottom of each page. Boxes with fixed position that
are larger than the page area are clipped. Parts of the fixed position
box that are not visible in the initial containing block will not
print.
Authors may use fixed positioning to create frame-like presentations.
Consider the following frame layout:
Image illustrating a frame-like layout with position='fixed'. [D]
This might be achieved with the following HTML document and style
rules:
A frame document with CSS 2.1
...
...
...
9.7 Relationships between 'display', 'position', and 'float'
The three properties that affect box generation and layout --
'display', 'position', and 'float' -- interact as follows:
1. If 'display' has the value 'none', then 'position' and 'float' do
not apply. In this case, the element generates no box.
2. Otherwise, if 'position' has the value 'absolute' or 'fixed', the
box is absolutely positioned, the computed value of 'float' is
'none', and display is set according to the table below. The
position of the box will be determined by the 'top', 'right',
'bottom' and 'left' properties and the box's containing block.
3. Otherwise, if 'float' has a value other than 'none', the box is
floated and 'display' is set according to the table below.
4. Otherwise, if the element is the root element, 'display' is set
according to the table below.
5. Otherwise, the remaining 'display' property values apply as
specified.
Specified value Computed value
inline-table table
inline, run-in, table-row-group, table-column, table-column-group,
table-header-group, table-footer-group, table-row, table-cell,
table-caption, inline-block block
others same as specified
9.8 Comparison of normal flow, floats, and absolute positioning
To illustrate the differences between normal flow, relative
positioning, floats, and absolute positioning, we provide a series of
examples based on the following HTML:
Comparison of positioning schemes
Beginning of body contents.
Start of outer contents.
Inner contents.
End of outer contents.
End of body contents.
In this document, we assume the following rules:
body { display: block; font-size:12px; line-height: 200%;
width: 400px; height: 400px }
p { display: block }
span { display: inline }
The final positions of boxes generated by the outer and inner elements
vary in each example. In each illustration, the numbers to the left of
the illustration indicate the normal flow position of the double-spaced
(for clarity) lines.
Note. The diagrams in this section are illustrative and not to scale.
They are meant to highlight the differences between the various
positioning schemes in CSS 2.1, and are not intended to be reference
renderings of the examples given.
9.8.1 Normal flow
Consider the following CSS declarations for outer and inner that don't
alter the normal flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and
two SPAN elements. Therefore, all of the content will be laid out in an
inline formatting context, within a containing block established by the
P element, producing something like:
Image illustrating the normal flow of text between parent and sibling
boxes. [D]
9.8.2 Relative positioning
To see the effect of relative positioning, we specify:
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The outer text is then
flowed into its normal flow position and dimensions at the end of line
1. Then, the inline boxes containing the text (distributed over three
lines) are shifted as a unit by '-12px' (upwards).
The contents of inner, as a child of outer, would normally flow
immediately after the words "of outer contents" (on line 1.5). However,
the inner contents are themselves offset relative to the outer contents
by '12px' (downwards), back to their original position on line 2.
Note that the content following outer is not affected by the relative
positioning of outer.
Image illustrating the effects of relative positioning on a box's
content. [D]
Note also that had the offset of outer been '-24px', the text of outer
and the body text would have overlapped.
9.8.3 Floating a box
Now consider the effect of floating the inner element's text to the
right by means of the following rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled out of the
flow and floated to the right margin (its 'width' has been assigned
explicitly). Line boxes to the left of the float are shortened, and the
document's remaining text flows into them.
Image illustrating the effects of floating a box. [D]
To show the effect of the 'clear' property, we add a sibling element to
the example:
Comparison of positioning schemes II
Beginning of body contents.
Start of outer contents.
Inner contents. Sibling contents.
End of outer contents.
End of body contents.
The following rules:
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
cause the inner box to float to the right as before and the document's
remaining text to flow into the vacated space:
Image illustrating the effects of floating a box without setting the
clear property to control the flow of text around the box. [D]
However, if the 'clear' property on the sibling element is set to
'right' (i.e., the generated sibling box will not accept a position
next to floating boxes to its right), the sibling content begins to
flow below the float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
Image illustrating the effects of floating an element with setting the
clear property to control the flow of text around the element. [D]
9.8.4 Absolute positioning
Finally, we consider the effect of absolute positioning. Consider the
following CSS declarations for outer and inner:
#outer {
position: absolute;
top: 200px; left: 200px;
width: 200px;
color: red;
}
#inner { color: blue }
which cause the top of the outer box to be positioned with respect to
its containing block. The containing block for a positioned box is
established by the nearest positioned ancestor (or, if none exists, the
initial containing block, as in our example). The top side of the outer
box is '200px' below the top of the containing block and the left side
is '200px' from the left side. The child box of outer is flowed
normally with respect to its parent.
Image illustrating the effects of absolutely positioning a box. [D]
The following example shows an absolutely positioned box that is a
child of a relatively positioned box. Although the parent outer box is
not actually offset, setting its 'position' property to 'relative'
means that its box may serve as the containing block for positioned
descendants. Since the outer box is an inline box that is split across
several lines, the first inline box's top and left edges (depicted by
thick dashed lines in the illustration below) serve as references for
'top' and 'left' offsets.
#outer {
position: relative;
color: red
}
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
This results in something like the following:
Image illustrating the effects of absolutely positioning a box with
respect to a containing block. [D]
If we do not position the outer box:
#outer { color: red }
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
the containing block for inner becomes the initial containing block (in
our example). The following illustration shows where the inner box
would end up in this case.
Image illustrating the effects of absolutely positioning a box with
respect to a containing block established by a normally positioned
parent. [D]
Relative and absolute positioning may be used to implement change bars,
as shown in the following example. The following fragment:
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
--
word.
might result in something like:
Image illustrating the use of floats to create a changebar effect.
[D]
First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset '10px' from the
left edge of the containing block (thus, a right margin of '10px' has
been reserved in anticipation of the offset). The two hyphens acting as
change bars are taken out of the flow and positioned at the current
line (due to 'top: auto'), '-1em' from the left edge of its containing
block (established by the P in its final position). The result is that
the change bars seem to "float" to the left of the current line.
9.9 Layered presentation
9.9.1 Specifying the stack level: the 'z-index' property
'z-index'
Value: auto | | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
For a positioned box, the 'z-index' property specifies:
1. The stack level of the box in the current stacking context.
2. Whether the box establishes a local stacking context.
Values have the following meanings:
This integer is the stack level of the generated box in the
current stacking context. The box also establishes a local
stacking context in which its stack level is '0'.
auto
The stack level of the generated box in the current stacking
context is the same as its parent's box. The box does not
establish a new local stacking context.
In this section, the expression "in front of" means closer to the user
as the user faces the screen.
In CSS 2.1, each box has a position in three dimensions. In addition to
their horizontal and vertical positions, boxes lie along a "z-axis" and
are formatted one on top of the other. Z-axis positions are
particularly relevant when boxes overlap visually. This section
discusses how boxes may be positioned along the z-axis.
The order in which the rendering tree is painted onto the canvas is
described in terms of stacking contexts. Stacking contexts can contain
further stacking contexts. A stacking context is atomic from the point
of view of its parent stacking context; boxes in other stacking
contexts may not come between any of its boxes.
Each box belongs to one stacking context. Each box in a given stacking
context has an integer stack level, which is its position on the z-axis
relative to other boxes in the same stacking context. Boxes with
greater stack levels are always formatted in front of boxes with lower
stack levels. Boxes may have negative stack levels. Boxes with the same
stack level in a stacking context are stacked back-to-front according
to document tree order.
The root element forms the root stacking context. Other stacking
contexts are generated by any positioned element (including relatively
positioned elements) having a computed value of 'z-index' other than
'auto'. Stacking contexts are not necessarily related to containing
blocks. In future levels of CSS, other properties may introduce
stacking contexts, for example 'opacity' [CSS3COLOR].
Each stacking context consists of the following stacking levels (from
back to front):
1. the background and borders of the element forming the stacking
context.
2. the stacking contexts of descendants with negative stack levels.
3. a stacking level containing in-flow non-inline-level non-positioned
descendants.
4. a stacking level for non-positioned floats and their contents.
5. a stacking level for in-flow inline-level non-positioned
descendants.
6. a stacking level for positioned descendants with 'z-index: auto',
and any descendant stacking contexts with 'z-index: 0'.
7. the stacking contexts of descendants with positive stack levels.
For a more thorough explanation of the stacking order, please see
Appendix E.
The contents of inline blocks and inline tables are stacked as if they
generated new stacking contexts, except that any positioned elements
and any elements that actually create new stacking contexts take part
in the parent stacking context. They are then painted atomically in the
inline stacking level.
In the following example, the stack levels of the boxes (named with
their "id" attributes) are: "text2"=0, "image"=1, "text3"=2, and
"text1"=3. The "text2" stack level is inherited from the root box. The
others are specified with the 'z-index' property.
Z-order positioning
This text will overlay the butterfly image.
This text will be beneath everything.
This text will underlay text1, but overlay the butterfly image
This example demonstrates the notion of transparency. The default
behavior of the background is to allow boxes behind it to be visible.
In the example, each box transparently overlays the boxes below it.
This behavior can be overridden by using one of the existing background
properties.
9.10 Text direction: the 'direction' and 'unicode-bidi' properties
Conforming user agents that do not support bidirectional text may
ignore the 'direction' and 'unicode-bidi' properties described in this
section. This exception includes UAs that render right-to-left
characters simply because a font on the system contains them but do not
support the concept of right-to-left text direction.
The characters in certain scripts are written from right to left. In
some documents, in particular those written with the Arabic or Hebrew
script, and in some mixed-language contexts, text in a single (visually
displayed) block may appear with mixed directionality. This phenomenon
is called bidirectionality, or "bidi" for short.
The Unicode standard ([UNICODE], section 3.11) defines a complex
algorithm for determining the proper directionality of text. The
algorithm consists of an implicit part based on character properties,
as well as explicit controls for embeddings and overrides. CSS 2.1
relies on this algorithm to achieve proper bidirectional rendering. The
'direction' and 'unicode-bidi' properties allow authors to specify how
the elements and attributes of a document language map to this
algorithm.
User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline boxes uninterrupted
by a forced line break or block boundary. This sequence forms the
"paragraph" unit in the bidirectional algorithm. The paragraph
embedding level is set according to the value of the 'direction'
property of the containing block rather than by the heuristic given in
steps P2 and P3 of the Unicode algorithm.
Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (DTDs),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them.
The HTML 4 specification ([HTML4], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet rules that
would achieve the bidi behavior specified in [HTML4] are given in the
sample style sheet. The HTML 4 specification also contains more
information on bidirectionality issues.
'direction'
Value: ltr | rtl | inherit
Initial: ltr
Applies to: all elements, but see prose
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the base writing direction of blocks and the
direction of embeddings and overrides (see 'unicode-bidi') for the
Unicode bidirectional algorithm. In addition, it specifies the
direction of table column layout, the direction of horizontal overflow,
and the position of an incomplete last line in a block in case of
'text-align: justify'.
Values for this property have the following meanings:
ltr
Left-to-right direction.
rtl
Right-to-left direction.
For the 'direction' property to affect reordering in inline-level
elements, the 'unicode-bidi' property's value must be 'embed' or
'override'.
Note. The 'direction' property, when specified for table column
elements, is not inherited by cells in the column since columns are not
the ancestors of the cells in the document tree. Thus, CSS cannot
easily capture the "dir" attribute inheritance rules described in
[HTML4], section 11.3.2.1.
'unicode-bidi'
Value: normal | embed | bidi-override | inherit
Initial: normal
Applies to: all elements, but see prose
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Values for this property have the following meanings:
normal
The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline-level
elements, implicit reordering works across element boundaries.
embed
If the element is inline-level, this value opens an additional
level of embedding with respect to the bidirectional algorithm.
The direction of this embedding level is given by the
'direction' property. Inside the element, reordering is done
implicitly. This corresponds to adding a LRE (U+202A; for
'direction: ltr') or RLE (U+202B; for 'direction: rtl') at the
start of the element and a PDF (U+202C) at the end of the
element.
bidi-override
For inline-level elements this creates an override. For
block-level, table-cell, table-caption, or inline-block elements
this creates an override for inline-level descendents not within
another block-level, table-cell, table-caption, or inline-block
element. This means that inside the element, reordering is
strictly in sequence according to the 'direction' property; the
implicit part of the bidirectional algorithm is ignored. This
corresponds to adding a LRO (U+202D; for 'direction: ltr') or
RLO (U+202E; for 'direction: rtl') at the start of the element
or at the start of each anonymous child block box, if any, and a
PDF (U+202C) at the end of the element.
The final order of characters in each block-level element is the same
as if the bidi control codes had been added as described above, markup
had been stripped, and the resulting character sequence had been passed
to an implementation of the Unicode bidirectional algorithm for plain
text that produced the same line-breaks as the styled text. In this
process, non-textual entities such as images are treated as neutral
characters, unless their 'unicode-bidi' property has a value other than
'normal', in which case they are treated as strong characters in the
'direction' specified for the element.
Please note that in order to be able to flow inline boxes in a uniform
direction (either entirely left-to-right or entirely right-to-left),
more inline boxes (including anonymous inline boxes) may have to be
created, and some inline boxes may have to be split up and reordered
before flowing.
Because the Unicode algorithm has a limit of 61 levels of embedding,
care should be taken not to use 'unicode-bidi' with a value other than
'normal' unless appropriate. In particular, a value of 'inherit' should
be used with extreme caution. However, for elements that are, in
general, intended to be displayed as blocks, a setting of
'unicode-bidi: embed' is preferred to keep the element together in case
display is changed to inline (see example below).
The following example shows an XML document with bidirectional text. It
illustrates an important design principle: DTD designers should take
bidi into account both in the language proper (elements and attributes)
and in any accompanying style sheets. The style sheets should be
designed so that bidi rules are separate from other style rules. The
bidi rules should not be overridden by other style sheets so that the
document language's or DTD's bidi behavior is preserved.
Example(s):
In this example, lowercase letters stand for inherently left-to-right
characters and uppercase letters represent inherently right-to-left
characters:
HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5HEBREW6 HEBREW7 HEBREW8english9 english10 english11 HEBREW12 HEBREW13english14 english15 english16english17 HEBREW18 english19 HEBREW20
Since this is XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:
/* Rules for bidi */
HEBREW, HE-QUO {direction: rtl; unicode-bidi: embed}
ENGLISH {direction: ltr; unicode-bidi: embed}
/* Rules for presentation */
HEBREW, ENGLISH, PAR {display: block}
EMPH {font-weight: bold}
The HEBREW element is a block with a right-to-left base direction, the
ENGLISH element is a block with a left-to-right base direction. The
PARs are blocks that inherit the base direction from their parents.
Thus, the first two PARs are read starting at the top right, the final
three are read starting at the top left. Please note that HEBREW and
ENGLISH are chosen as element names for explicitness only; in general,
element names should convey structure without reference to language.
The EMPH element is inline-level, and since its value for
'unicode-bidi' is 'normal' (the initial value), it has no effect on the
ordering of the text. The HE-QUO element, on the other hand, creates an
embedding.
The formatting of this text might look like this if the line length is
long:
5WERBEH 4WERBEH english3 2WERBEH 1WERBEH
8WERBEH 7WERBEH 6WERBEH
english9 english10 english11 13WERBEH 12WERBEH
english14 english15 english16
english17 20WERBEH english19 18WERBEH
Note that the HE-QUO embedding causes HEBREW18 to be to the right of
english19.
If lines have to be broken, it might be more like this:
2WERBEH 1WERBEH
-EH 4WERBEH english3
5WERB
-EH 7WERBEH 6WERBEH
8WERB
english9 english10 en-
glish11 12WERBEH
13WERBEH
english14 english15
english16
english17 18WERBEH
20WERBEH english19
Because HEBREW18 must be read before english19, it is on the line above
english19. Just breaking the long line from the earlier formatting
would not have worked. Note also that the first syllable from english19
might have fit on the previous line, but hyphenation of left-to-right
words in a right-to-left context, and vice versa, is usually suppressed
to avoid having to display a hyphen in the middle of a line.
__________________________________________________________________
__________________________________________________________________
10 Visual formatting model details
Contents
* 10.1 Definition of "containing block"
* 10.2 Content width: the 'width' property
* 10.3 Calculating widths and margins
+ 10.3.1 Inline, non-replaced elements
+ 10.3.2 Inline, replaced elements
+ 10.3.3 Block-level, non-replaced elements in normal flow
+ 10.3.4 Block-level, replaced elements in normal flow
+ 10.3.5 Floating, non-replaced elements
+ 10.3.6 Floating, replaced elements
+ 10.3.7 Absolutely positioned, non-replaced elements
+ 10.3.8 Absolutely positioned, replaced elements
+ 10.3.9 'Inline-block', non-replaced elements in normal flow
+ 10.3.10 'Inline-block', replaced elements in normal flow
* 10.4 Minimum and maximum widths: 'min-width' and 'max-width'
* 10.5 Content height: the 'height' property
* 10.6 Calculating heights and margins
+ 10.6.1 Inline, non-replaced elements
+ 10.6.2 Inline replaced elements, block-level replaced elements
in normal flow, 'inline-block' replaced elements in normal
flow and floating replaced elements
+ 10.6.3 Block-level non-replaced elements in normal flow when
'overflow' computes to 'visible'
+ 10.6.4 Absolutely positioned, non-replaced elements
+ 10.6.5 Absolutely positioned, replaced elements
+ 10.6.6 Complicated cases
+ 10.6.7 'Auto' heights for block formatting context roots
* 10.7 Minimum and maximum heights: 'min-height' and 'max-height'
* 10.8 Line height calculations: the 'line-height' and
'vertical-align' properties
+ 10.8.1 Leading and half-leading
10.1 Definition of "containing block"
The position and size of an element's box(es) are sometimes calculated
relative to a certain rectangle, called the containing block of the
element. The containing block of an element is defined as follows:
1. The containing block in which the root element lives is a rectangle
called the initial containing block. For continuous media, it has
the dimensions of the viewport and is anchored at the canvas
origin; it is the page area for paged media. The 'direction'
property of the initial containing block is the same as for the
root element.
2. For other elements, if the element's position is 'relative' or
'static', the containing block is formed by the content edge of the
nearest block-level, table cell or inline-block ancestor box.
3. If the element has 'position: fixed', the containing block is
established by the viewport in the case of continuous media or the
page area in the case of paged media.
4. If the element has 'position: absolute', the containing block is
established by the nearest ancestor with a 'position' of
'absolute', 'relative' or 'fixed', in the following way:
1. In the case that the ancestor is inline-level, the containing
block depends on the 'direction' property of the ancestor:
1. If the 'direction' is 'ltr', the top and left of the
containing block are the top and left padding edges of
the first box generated by the ancestor, and the bottom
and right are the bottom and right padding edges of the
last box of the ancestor.
2. If the 'direction' is 'rtl', the top and right are the
top and right padding edges of the first box generated by
the ancestor, and the bottom and left are the bottom and
left padding edges of the last box of the ancestor.
Note: This may cause the containing block's width to be
negative.
2. Otherwise, the containing block is formed by the padding edge
of the ancestor.
If there is no such ancestor, the containing block is the initial
containing block.
In paged media, an absolutely positioned element is positioned relative
to its containing block ignoring any page breaks (as if the document
were continuous). The element may subsequently be broken over several
pages.
For absolutely positioned content that resolves to a position on a page
other than the page being laid out (the current page), or resolves to a
position on the current page which has already been rendered for
printing, printers may place the content
* on another location on the current page,
* on a subsequent page, or
* may omit it.
Note that a block-level element that is split over several pages may
have a different width on each page and that there may be
device-specific limits.
Example(s):
With no positioning, the containing blocks (C.B.) in the following
document:
Illustration of containing blocks
This is text in the first paragraph...
This is text in the
second paragraph.
are established as follows:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 body
p1 div1
p2 div1
em1 p2
strong1 p2
If we position "div1":
#div1 { position: absolute; left: 50px; top: 50px }
its containing block is no longer "body"; it becomes the initial
containing block (since there are no other positioned ancestor boxes).
If we position "em1" as well:
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
the table of containing blocks becomes:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 initial C.B.
p1 div1
p2 div1
em1 div1
strong1 em1
By positioning "em1", its containing block becomes the nearest
positioned ancestor box (i.e., that generated by "div1").
10.2 Content width: the 'width' property
'width'
Value: | | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage or 'auto' as specified or the absolute
length; 'auto' if the property does not apply
This property specifies the content width of boxes generated by
block-level and replaced elements.
This property does not apply to non-replaced inline-level elements. The
content width of a non-replaced inline element's boxes is that of the
rendered content within them (before any relative offset of children).
Recall that inline boxes flow into line boxes. The width of line boxes
is given by the their containing block, but may be shorted by the
presence of floats.
The width of a replaced element's box is intrinsic and may be scaled by
the user agent if the value of this property is different than 'auto'.
Values have the following meanings:
Specifies the width of the content area using a length unit.
Specifies a percentage width. The percentage is calculated with
respect to the width of the generated box's containing block. If
the containing block's width depends on this element's width,
then the resulting layout is undefined in CSS 2.1. Note: For
absolutely positioned elements whose containing block is based
on a block-level element, the percentage is calculated with
respect to the width of the padding box of that element. This is
a change from CSS1, where the percentage width was always
calculated with respect to the content box of the parent
element.
auto
The width depends on the values of other properties. See the
sections below.
Negative values for 'width' are illegal.
Example(s):
For example, the following rule fixes the content width of paragraphs
at 100 pixels:
p { width: 100px }
10.3 Calculating widths and margins
The values of an element's 'width', 'margin-left', 'margin-right',
'left' and 'right' properties as used for layout depend on the type of
box generated and on each other. (The value used for layout is
sometimes referred to as the used value.) In principle, the values used
are the same as the computed values, with 'auto' replaced by some
suitable value, and percentages calculated based on the containing
block, but there are exceptions. The following situations need to be
distinguished:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
For Points 1-6 and 9-10, the values of 'left' and 'right' in the case
of relatively positioned elements are determined by the rules in
section 9.4.3.
10.3.1 Inline, non-replaced elements
The 'width' property does not apply. A computed value of 'auto' for
'margin-left' or 'margin-right' becomes a used value of '0'.
10.3.2 Inline, replaced elements
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes
a used value of '0'.
If 'height' and 'width' both have computed values of 'auto' and the
element also has an intrinsic width, then that intrinsic width is the
used value of 'width'.
If 'height' and 'width' both have computed values of 'auto' and the
element has no intrinsic width, but does have an intrinsic height and
intrinsic ratio; or if 'width' has a computed value of 'auto', 'height'
has some other computed value, and the element does have an intrinsic
ratio; then the used value of 'width' is:
(used height) * (intrinsic ratio)
If 'height' and 'width' both have computed values of 'auto' and the
element has an intrinsic ratio but no intrinsic height or width and the
containing block's width doesn't itself depend on the replaced
element's width, then the used value of 'width' is calculated from the
constraint equation used for block-level, non-replaced elements in
normal flow.
Otherwise, if 'width' has a computed value of 'auto', and the element
has an intrinsic width, then that intrinsic width is the used value of
'width'.
Otherwise, if 'width' has a computed value of 'auto', but none of the
conditions above are met, then the used value of 'width' becomes 300px.
If 300px is too wide to fit the device, UAs should use the width of the
largest rectangle that has a 2:1 ratio and fits the device instead.
Percentage intrinsic widths are first evaluated with respect to the
containing block's width, if that width doesn't itself depend on the
replaced element's width. If it does, then the resulting layout is
undefined in CSS 2.1.
10.3.3 Block-level, non-replaced elements in normal flow
The following constraints must hold among the used values of the other
properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + 'margin-right' = width of
containing block
If 'width' is not 'auto' and 'border-left-width' + 'padding-left' +
'width' + 'padding-right' + 'border-right-width' (plus any of
'margin-left' or 'margin-right' that are not 'auto') is larger than the
width of the containing block, then any 'auto' values for 'margin-left'
or 'margin-right' are, for the following rules, treated as zero.
If all of the above have a computed value other than 'auto', the values
are said to be "over-constrained" and one of the used values will have
to be different from its computed value. If the 'direction' property of
the containing block has the value 'ltr', the specified value of
'margin-right' is ignored and the value is calculated so as to make the
equality true. If the value of 'direction' is 'rtl', this happens to
'margin-left' instead.
If there is exactly one value specified as 'auto', its used value
follows from the equality.
If 'width' is set to 'auto', any other 'auto' values become '0' and
'width' follows from the resulting equality.
If both 'margin-left' and 'margin-right' are 'auto', their used values
are equal. This horizontally centers the element with respect to the
edges of the containing block.
10.3.4 Block-level, replaced elements in normal flow
The used value of 'width' is determined as for inline replaced
elements. Then the rules for non-replaced block-level elements are
applied to determine the margins.
10.3.5 Floating, non-replaced elements
If 'margin-left', or 'margin-right' are computed as 'auto', their used
value is '0'.
If 'width' is computed as 'auto', the used value is the "shrink-to-fit"
width.
Calculation of the shrink-to-fit width is similar to calculating the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur, and
also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, find the available width: in this case, this is the width of
the containing block minus the used values of 'margin-left',
'border-left-width', 'padding-left', 'padding-right',
'border-right-width', 'margin-right', and the widths of any relevant
scroll bars.
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
10.3.6 Floating, replaced elements
If 'margin-left' or 'margin-right' are computed as 'auto', their used
value is '0'. The used value of 'width' is determined as for inline
replaced elements.
10.3.7 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely:
* The static-position containing block is the containing block of a
hypothetical box that would have been the first box of the element
if its specified 'position' value had been 'static' and its
specified 'float' had been 'none'. (Note that due to the rules in
section 9.7 this hypothetical calculation might require also
assuming a different computed value for 'display'.)
* The static position for 'left' is the distance from the left edge
of the containing block to the left margin edge of a hypothetical
box that would have been the first box of the element if its
'position' property had been 'static' and 'float' had been 'none'.
The value is negative if the hypothetical box is to the left of the
containing block.
* The static position for 'right' is the distance from the right edge
of the containing block to the right margin edge of the same
hypothetical box as above. The value is positive if the
hypothetical box is to the left of the containing block's edge.
But rather than actually calculating the dimensions of that
hypothetical box, user agents are free to make a guess at its probable
position.
For the purposes of calculating the static position, the containing
block of fixed positioned elements is the initial containing block
instead of the viewport, and all scrollable boxes should be assumed to
be scrolled to their origin.
The constraint that determines the used values for these elements is:
'left' + 'margin-left' + 'border-left-width' + 'padding-left' +
'width' + 'padding-right' + 'border-right-width' + 'margin-right' +
'right' = width of containing block
If all three of 'left', 'width', and 'right' are 'auto': First set any
'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the
'direction' property of the element establishing the static-position
containing block is 'ltr' set 'left' to the static position and apply
rule number three below; otherwise, set 'right' to the static position
and apply rule number one below.
If none of the three is 'auto': If both 'margin-left' and
'margin-right' are 'auto', solve the equation under the extra
constraint that the two margins get equal values, unless this would
make them negative, in which case when direction of the containing
block is 'ltr' ('rtl'), set 'margin-left' ('margin-right') to zero and
solve for 'margin-right' ('margin-left'). If one of 'margin-left' or
'margin-right' is 'auto', solve the equation for that value. If the
values are over-constrained, ignore the value for 'left' (in case the
'direction' property of the containing block is 'rtl') or 'right' (in
case 'direction' is 'ltr') and solve for that value.
Otherwise, set 'auto' values for 'margin-left' and 'margin-right' to 0,
and pick the one of the following six rules that applies.
1. 'left' and 'width' are 'auto' and 'right' is not 'auto', then the
width is shrink-to-fit. Then solve for 'left'
2. 'left' and 'right' are 'auto' and 'width' is not 'auto', then if
the 'direction' property of the element establishing the
static-position containing block is 'ltr' set 'left' to the static
position, otherwise set 'right' to the static position. Then solve
for 'left' (if 'direction is 'rtl') or 'right' (if 'direction' is
'ltr').
3. 'width' and 'right' are 'auto' and 'left' is not 'auto', then the
width is shrink-to-fit . Then solve for 'right'
4. 'left' is 'auto', 'width' and 'right' are not 'auto', then solve
for 'left'
5. 'width' is 'auto', 'left' and 'right' are not 'auto', then solve
for 'width'
6. 'right' is 'auto', 'left' and 'width' are not 'auto', then solve
for 'right'
Calculation of the shrink-to-fit width is similar to calculating the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur, and
also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, calculate the available width: this is found by solving for
'width' after setting 'left' (in case 1) or 'right' (in case 3) to 0.
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
10.3.8 Absolutely positioned, replaced elements
In this case, section 10.3.7 applies up through and including the
constraint equation, but the rest of section 10.3.7 is replaced by the
following rules:
1. The used value of 'width' is determined as for inline replaced
elements. If 'margin-left' or 'margin-right' is specified as 'auto'
its used value is determined by the rules below.
2. If both 'left' and 'right' have the value 'auto', then if the
'direction' property of the element establishing the
static-position containing block is 'ltr', set 'left' to the static
position; else if 'direction' is 'rtl', set 'right' to the static
position.
3. If 'left' or 'right' are 'auto', replace any 'auto' on
'margin-left' or 'margin-right' with '0'.
4. If at this point both 'margin-left' and 'margin-right' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values, unless this would make them
negative, in which case when the direction of the containing block
is 'ltr' ('rtl'), set 'margin-left' ('margin-right') to zero and
solve for 'margin-right' ('margin-left').
5. If at this point there is an 'auto' left, solve the equation for
that value.
6. If at this point the values are over-constrained, ignore the value
for either 'left' (in case the 'direction' property of the
containing block is 'rtl') or 'right' (in case 'direction' is
'ltr') and solve for that value.
10.3.9 'Inline-block', non-replaced elements in normal flow
If 'width' is 'auto', the used value is the shrink-to-fit width as for
floating elements.
A computed value of 'auto' for 'margin-left' or 'margin-right' becomes
a used value of '0'.
10.3.10 'Inline-block', replaced elements in normal flow
Exactly as inline replaced elements.
10.4 Minimum and maximum widths: 'min-width' and 'max-width'
'min-width'
Value: | | inherit
Initial: 0
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
'max-width'
Value: | | none | inherit
Initial: none
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length or
'none'
These two properties allow authors to constrain content widths to a
certain range. Values have the following meanings:
Specifies a fixed minimum or maximum used width.
Specifies a percentage for determining the used value. The
percentage is calculated with respect to the width of the
generated box's containing block. If the containing block's
width is negative, the used value is zero. If the containing
block's width depends on this element's width, then the
resulting layout is undefined in CSS 2.1.
none
(Only on 'max-width') No limit on the width of the box.
Negative values for 'min-width' and 'max-width' are illegal.
The following algorithm describes how the two properties influence the
used value of the 'width' property:
1. The tentative used width is calculated (without 'min-width' and
'max-width') following the rules under "Calculating widths and
margins" above.
2. If the tentative used width is greater than 'max-width', the rules
above are applied again, but this time using the computed value of
'max-width' as the computed value for 'width'.
3. If the resulting width is smaller than 'min-width', the rules above
are applied again, but this time using the value of 'min-width' as
the computed value for 'width'.
These steps do not affect the real computed values of the above
properties.
However, for replaced elements with an intrinsic ratio and both 'width'
and 'height' specified as 'auto', the algorithm is as follows:
Select from the table the resolved height and width values for the
appropriate constraint violation. Take the max-width and max-height as
max(min, max) so that min <= max holds true. In this table w and h
stand for the results of the width and height computations ignoring the
'min-width', 'min-height', 'max-width' and 'max-height' properties.
Normally these are the intrinsic width and height, but they may not be
in the case of replaced elements with intrinsic ratios.
Note: In cases where an explicit width or height is set and the other
dimension is auto, applying a minimum or maximum constraint on the auto
side can cause an over-constrained situation. The spec is clear in the
behavior but it might not be what the author expects. The CSS3
image-fit property can be used to obtain different results in this
situation.
Constraint Violation Resolved Width Resolved Height
none w h
w > max-width max-width max(max-width * h/w, min-height)
w < min-width min-width min(min-width * h/w, max-height)
h > max-height max(max-height * w/h, min-width) max-height
h < min-height min(min-height * w/h, max-width) min-height
(w > max-width) and (h > max-height), where
(max-width/w <= max-height/h) max-width max(min-height,
max-width * h/w)
(w > max-width) and (h > max-height), where
(max-width/w > max-height/h) max(min-width, max-height * w/h)
max-height
(w < min-width) and (h < min-height), where
(min-width/w <= min-height/h) min(max-width, min-height * w/h)
min-height
(w < min-width) and (h < min-height), where
(min-width/w > min-height/h) min-width min(max-height, min-width * h/w)
(w < min-width) and (h > max-height) min-width max-height
(w > max-width) and (h < min-height) max-width min-height
Then apply the rules under "Calculating widths and margins" above, as
if 'width' were computed as this value.
10.5 Content height: the 'height' property
'height'
Value: | | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage or 'auto' (see prose under
) or the absolute length; 'auto' if the property does not
apply
This property specifies the content height of boxes generated by
block-level, inline-block and replaced elements.
This property does not apply to non-replaced inline-level elements. See
the section on computing heights and margins for non-replaced inline
elements for the rules used instead.
Values have the following meanings:
Specifies the height of the content area using a length value.
Specifies a percentage height. The percentage is calculated with
respect to the height of the generated box's containing block.
If the height of the containing block is not specified
explicitly (i.e., it depends on content height), and this
element is not absolutely positioned, the value computes to
'auto'. A percentage height on the root element is relative to
the initial containing block. Note: For absolutely positioned
elements whose containing block is based on a block-level
element, the percentage is calculated with respect to the height
of the padding box of that element. This is a change from CSS1,
where the percentage was always calculated with respect to the
content box of the parent element.
auto
The height depends on the values of other properties. See the
prose below.
Note that the height of the containing block of an absolutely
positioned element is independent of the size of the element itself,
and thus a percentage height on such an element can always be resolved.
However, it may be that the height is not known until elements that
come later in the document have been processed.
Negative values for 'height' are illegal.
Example(s):
For example, the following rule sets the content height of paragraphs
to 100 pixels:
p { height: 100px }
Paragraphs of which the height of the contents exceeds 100 pixels will
overflow according to the 'overflow' property.
10.6 Calculating heights and margins
For calculating the values of 'top', 'margin-top', 'height',
'margin-bottom', and 'bottom' a distinction must be made between
various kinds of boxes:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
For Points 1-6 and 9-10, the used values of 'top' and 'bottom' are
determined by the rules in section 9.4.3.
Note: these rules apply to the root element just as to any other
element.
10.6.1 Inline, non-replaced elements
The 'height' property doesn't apply. The height of the content area
should be based on the font, but this specification does not specify
how. A UA may, e.g., use the em-box or the maximum ascender and
descender of the font. (The latter would ensure that glyphs with parts
above or below the em-box still fall within the content area, but leads
to differently sized boxes for different fonts; the former would ensure
authors can control background styling relative to the 'line-height',
but leads to glyphs painting outside their content area.)
Note: level 3 of CSS will probably include a property to select which
measure of the font is used for the content height.
The vertical padding, border and margin of an inline, non-replaced box
start at the top and bottom of the content area, not the 'line-height'.
But only the 'line-height' is used when calculating the height of the
line box.
If more than one font is used (this could happen when glyphs are found
in different fonts), the height of the content area is not defined by
this specification. However, we suggest that the height is chosen such
that the content area is just high enough for either (1) the em-boxes,
or (2) the maximum ascenders and descenders, of all the fonts in the
element. Note that this may be larger than any of the font sizes
involved, depending on the baseline alignment of the fonts.
10.6.2 Inline replaced elements, block-level replaced elements in normal flow,
'inline-block' replaced elements in normal flow and floating replaced elements
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' and 'width' both have computed values of 'auto' and the
element also has an intrinsic height, then that intrinsic height is the
used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto', and the element
has an intrinsic ratio then the used value of 'height' is:
(used width) / (intrinsic ratio)
Otherwise, if 'height' has a computed value of 'auto', and the element
has an intrinsic height, then that intrinsic height is the used value
of 'height'.
Otherwise, if 'height' has a computed value of 'auto', but none of the
conditions above are met, then the used value of 'height' must be set
to the height of the largest rectangle that has a 2:1 ratio, has a
height not greater than 150px, and has a width not greater than the
device width.
Percentage intrinsic heights are evaluated with respect to the
containing block's height, if that height is specified explicitly, or
if the replaced element is absolutely positioned. If neither of these
conditions is met, then percentage values on such replaced elements
can't be resolved and such elements are assumed to have no intrinsic
height.
For 'inline' and 'inline-block' elements, the margin box is used when
calculating the height of the line box.
10.6.3 Block-level non-replaced elements in normal flow when 'overflow'
computes to 'visible'
This section also applies to block-level non-replaced elements in
normal flow when 'overflow' does not compute to 'visible' but has been
propagated to the viewport.
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' is 'auto', the height depends on whether the element has
any block-level children and whether it has padding or borders:
If it only has inline-level children, the height is the distance
between the top of the topmost line box and the bottom of the
bottommost line box.
If it has block-level children, the height is the distance between the
top border-edge of the topmost block-level child box that doesn't have
margins collapsed through it and the bottom border-edge of the
bottommost block-level child box that doesn't have margins collapsed
through it. However, if the element has a non-zero top padding and/or
top border, or is the root element, then the content starts at the top
margin edge of the topmost child. (The first case expresses the fact
that the top and bottom margins of the element collapse with those of
the topmost and bottommost children, while in the second case the
presence of the padding/border prevents the top margins from
collapsing.) Similarly, if the bottom margin of the block does not
collapse with the bottom margin of its last in-flow child, then the
content ends at the bottom margin edge of the bottommost child.
Only children in the normal flow are taken into account (i.e., floating
boxes and absolutely positioned boxes are ignored, and relatively
positioned boxes are considered without their offset). Note that the
child box may be an anonymous block box.
10.6.4 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely, the static position
for 'top' is the distance from the top edge of the containing block to
the top margin edge of a hypothetical box that would have been the
first box of the element if its specified 'position' value had been
'static' and its specified 'float' had been 'none' and 'clear' had been
'none'. (Note that due to the rules in section 9.7 this might require
also assuming a different computed value for 'display'.) The value is
negative if the hypothetical box is above the containing block.
But rather than actually calculating the dimensions of that
hypothetical box, user agents are free to make a guess at its probable
position.
For the purposes of calculating the static position, the containing
block of fixed positioned elements is the initial containing block
instead of the viewport.
For absolutely positioned elements, the used values of the vertical
dimensions must satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height'
+ 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' +
'bottom' = height of containing block
If all three of 'top', 'height', and 'bottom' are auto, set 'top' to
the static position and apply rule number three below.
If none of the three are 'auto': If both 'margin-top' and
'margin-bottom' are 'auto', solve the equation under the extra
constraint that the two margins get equal values. If one of
'margin-top' or 'margin-bottom' is 'auto', solve the equation for that
value. If the values are over-constrained, ignore the value for
'bottom' and solve for that value.
Otherwise, pick the one of the following six rules that applies.
1. 'top' and 'height' are 'auto' and 'bottom' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'top'
2. 'top' and 'bottom' are 'auto' and 'height' is not 'auto', then set
'top' to the static position, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
3. 'height' and 'bottom' are 'auto' and 'top' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
4. 'top' is 'auto', 'height' and 'bottom' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve
for 'top'
5. 'height' is 'auto', 'top' and 'bottom' are not 'auto', then 'auto'
values for 'margin-top' and 'margin-bottom' are set to 0 and solve
for 'height'
6. 'bottom' is 'auto', 'top' and 'height' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0 and solve
for 'bottom'
10.6.5 Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element
has an intrinsic height. The sequence of substitutions is now:
1. The used value of 'height' is determined as for inline replaced
elements. If 'margin-top' or 'margin-bottom' is specified as 'auto'
its used value is determined by the rules below.
2. If both 'top' and 'bottom' have the value 'auto', replace 'top'
with the element's static position.
3. If 'bottom' is 'auto', replace any 'auto' on 'margin-top' or
'margin-bottom' with '0'.
4. If at this point both 'margin-top' and 'margin-bottom' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values.
5. If at this point there is only one 'auto' left, solve the equation
for that value.
6. If at this point the values are over-constrained, ignore the value
for 'bottom' and solve for that value.
10.6.6 Complicated cases
This section applies to:
* Block-level, non-replaced elements in normal flow when 'overflow'
does not compute to 'visible' (except if the 'overflow' property's
value has been propagated to the viewport).
* 'Inline-block', non-replaced elements.
* Floating, non-replaced elements.
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
If 'height' is 'auto', the height depends on the element's descendants.
For 'inline-block' elements, the margin box is used when calculating
the height of the line box.
10.6.7 'Auto' heights for block formatting context roots
In certain cases (see the preceding sections), the height of an element
is computed as follows:
If it only has inline-level children, the height is the distance
between the top of the topmost line box and the bottom of the
bottommost line box.
If it has block-level children, the height is the distance between the
top margin-edge of the topmost block-level child box and the bottom
margin-edge of the bottommost block-level child box.
Absolutely positioned children are ignored, and relatively positioned
boxes are considered without their offset. Note that the child box may
be an anonymous block box.
In addition, if the element has any floating descendants whose bottom
margin edge is below the bottom, then the height is increased to
include those edges. Only floats that are children of the element
itself or of descendants in the normal flow are taken into account,
e.g., floats inside absolutely positioned descendants or other floats
are not.
10.7 Minimum and maximum heights: 'min-height' and 'max-height'
It is sometimes useful to constrain the height of elements to a certain
range. Two properties offer this functionality:
'min-height'
Value: | | inherit
Initial: 0
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage as specified or the absolute length
'max-height'
Value: | | none | inherit
Initial: none
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: the percentage as specified or the absolute length or
'none'
These two properties allow authors to constrain box heights to a
certain range. Values have the following meanings:
Specifies a fixed minimum or maximum computed height.
Specifies a percentage for determining the used value. The
percentage is calculated with respect to the height of the
generated box's containing block. If the height of the
containing block is not specified explicitly (i.e., it depends
on content height), and this element is not absolutely
positioned, the percentage value is treated as '0' (for
'min-height') or 'none' (for 'max-height').
none
(Only on 'max-height') No limit on the height of the box.
Negative values for 'min-height' and 'max-height' are illegal.
The following algorithm describes how the two properties influence the
used value of the 'height' property:
1. The tentative used height is calculated (without 'min-height' and
'max-height') following the rules under "Calculating heights and
margins" above.
2. If this tentative height is greater than 'max-height', the rules
above are applied again, but this time using the value of
'max-height' as the computed value for 'height'.
3. If the resulting height is smaller than 'min-height', the rules
above are applied again, but this time using the value of
'min-height' as the computed value for 'height'.
These steps do not affect the real computed values of the above
properties. The change of used 'height' has no effect on margin
collapsing except as specifically required by rules for 'min-height' or
'max-height' in "Collapsing margins" (8.3.1).
However, for replaced elements with both 'width' and 'height' computed
as 'auto', use the algorithm under Minimum and maximum widths above to
find the used width and height. Then apply the rules under "Computing
heights and margins" above, using the resulting width and height as if
they were the computed values.
10.8 Line height calculations: the 'line-height' and 'vertical-align' properties
As described in the section on inline formatting contexts, user agents
flow inline boxes into a vertical stack of line boxes. The height of a
line box is determined as follows:
1. The height of each inline box in the line box is calculated (see
"Calculating heights and margins" and the 'line-height' property).
2. The inline boxes are aligned vertically according to their
'vertical-align' property.
3. The line box height is the distance between the uppermost box top
and the lowermost box bottom.
4. If the resulting height is smaller than the minimal height of line
boxes for this block, as specified by the 'line-height' property,
the height is increased to be that minimal height.
Empty inline elements generate empty inline boxes, but these boxes
still have margins, padding, borders and a line height, and thus
influence these calculations just like elements with content.
10.8.1 Leading and half-leading
Since the value of 'line-height' may be different from the height of
the content area there may be space above and below rendered glyphs.
The difference between the content height and the used value of
'line-height' is called the leading. Half the leading is called the
half-leading.
User agents center glyphs vertically in an inline box, adding
half-leading on the top and bottom. For example, if a piece of text is
'12px' high and the 'line-height' value is '14px', 2pxs of extra space
should be added: 1px above and 1px below the letters. (This applies to
empty boxes as well, as if the empty box contained an infinitely narrow
letter.)
When the 'line-height' value is less than the content height, the final
inline box height will be less than the font size and the rendered
glyphs will "bleed" outside the box. If such a box touches the edge of
a line box, the rendered glyphs will also "bleed" into the adjoining
line box.
Although margins, borders, and padding of non-replaced elements do not
enter into the line box calculation, they are still rendered around
inline boxes. This means that if the height specified by 'line-height'
is less than the content height of contained boxes, backgrounds and
colors of padding and borders may "bleed" into adjoining line boxes.
User agents should render the boxes in document order. This will cause
the borders on subsequent lines to paint over the borders and text of
previous lines.
'line-height'
Value: normal | | | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: refer to the font size of the element itself
Media: visual
Computed value: for and the absolute value;
otherwise as specified
On a block-level, table-cell, table-caption or inline-block element
whose content is composed of inline-level elements, 'line-height'
specifies the minimal height of line boxes within the element. The
minimum height consists of a minimum height above the block's baseline
and a minimum depth below it, exactly as if each line box starts with a
zero-width inline box with the block's font and line height properties
(what T[E]X calls a "strut").
On an inline-level element, 'line-height' specifies the height that is
used in the calculation of the line box height (except for inline
replaced elements, where the height of the box is given by the 'height'
property).
Values for this property have the following meanings:
normal
Tells user agents to set the used value to a "reasonable" value
based on the font of the element. The value has the same meaning
as . We recommend a used value for 'normal' between 1.0
to 1.2. The computed value is 'normal'.
The specified length is used in the calculation of the line box
height. Negative values are illegal.
The used value of the property is this number multiplied by the
element's font size. Negative values are illegal. The computed
value is the same as the specified value.
The computed value of the property is this percentage multiplied
by the element's computed font size. Negative values are
illegal.
Example(s):
The three rules in the example below have the same resultant line
height:
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
When an element contains text that is rendered in more than one font,
user agents may determine the 'line-height' value according to the
largest font size.
Generally, when there is only one value of 'line-height' for all inline
boxes in a paragraph (and no tall images), the above will ensure that
baselines of successive lines are exactly 'line-height' apart. This is
important when columns of text in different fonts have to be aligned,
for example in a table.
'vertical-align'
Value: baseline | sub | super | top | text-top | middle | bottom |
text-bottom | | | inherit
Initial: baseline
Applies to: inline-level and 'table-cell' elements
Inherited: no
Percentages: refer to the 'line-height' of the element itself
Media: visual
Computed value: for and the absolute length,
otherwise as specified
This property affects the vertical positioning inside a line box of the
boxes generated by an inline-level element.
Note. Values of this property have different meanings in the context of
tables. Please consult the section on table height algorithms for
details.
The following values only have meaning with respect to a parent
inline-level element, or to the strut of a parent block-level,
table-cell, table-caption or inline-block element.
baseline
Align the baseline of the box with the baseline of the parent
box. If the box doesn't have a baseline, align the bottom margin
edge with the parent's baseline.
middle
Align the vertical midpoint of the box with the baseline of the
parent box plus half the x-height of the parent.
sub
Lower the baseline of the box to the proper position for
subscripts of the parent's box. (This value has no effect on the
font size of the element's text.)
super
Raise the baseline of the box to the proper position for
superscripts of the parent's box. (This value has no effect on
the font size of the element's text.)
text-top
Align the top of the box with the top of the parent's content
area (see 10.6.1).
text-bottom
Align the bottom of the box with the bottom of the parent's
content area (see 10.6.1).
Raise (positive value) or lower (negative value) the box by this
distance (a percentage of the 'line-height' value). The value
'0%' means the same as 'baseline'.
Raise (positive value) or lower (negative value) the box by this
distance. The value '0cm' means the same as 'baseline'.
The following values align the element relative to the line box. Since
the element may have children aligned relative to it (which in turn may
have descendants aligned relative to them), these values use the bounds
of the aligned subtree. The aligned subtree of an inline element
contains that element and the aligned subtrees of all children inline
elements whose computed 'vertical-align' value is not 'top' or
'bottom'. The top of the aligned subtree is the highest of the tops of
the boxes in the subtree, and the bottom is analogous.
top
Align the top of the aligned subtree with the top of the line
box.
bottom
Align the bottom of the aligned subtree with the bottom of the
line box.
The baseline of an 'inline-table' is the baseline of the first row of
the table.
The baseline of an 'inline-block' is the baseline of its last line box
in the normal flow, unless it has either no in-flow line boxes or if
its 'overflow' property has a computed value other than 'visible', in
which case the baseline is the bottom margin edge.
__________________________________________________________________
__________________________________________________________________
11 Visual effects
Contents
* 11.1 Overflow and clipping
+ 11.1.1 Overflow: the 'overflow' property
+ 11.1.2 Clipping: the 'clip' property
* 11.2 Visibility: the 'visibility' property
11.1 Overflow and clipping
Generally, the content of a block box is confined to the content edges
of the box. In certain cases, a box may overflow, meaning its content
lies partly or entirely outside of the box, e.g.:
* A line cannot be broken, causing the line box to be wider than the
block box.
* A block-level box is too wide for the containing block. This may
happen when an element's 'width' property has a value that causes
the generated block box to spill over sides of the containing
block.
* An element's height exceeds an explicit height assigned to the
containing block (i.e., the containing block's height is determined
by the 'height' property, not by content height).
* A descendent box is positioned absolutely, partly outside the box.
Such boxes are not always clipped by the overflow property on their
ancestors.
* A descendent box has negative margins, causing it to be positioned
partly outside the box.
* The 'text-indent' property causes an inline box to hang off either
the left or right edge of the block box.
Whenever overflow occurs, the 'overflow' property specifies whether a
box is clipped to its padding edge, and if so, whether a scrolling
mechanism is provided to access any clipped out content.
11.1.1 Overflow: the 'overflow' property
'overflow'
Value: visible | hidden | scroll | auto | inherit
Initial: visible
Applies to: non-replaced block-level elements, table cells, and
inline-block elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether content of a block-level element is
clipped when it overflows the element's box. It affects the clipping of
all of the element's content except any descendant elements (and their
respective content and descendants) whose containing block is the
viewport or an ancestor of the element. Values have the following
meanings:
visible
This value indicates that content is not clipped, i.e., it may
be rendered outside the block box.
hidden
This value indicates that the content is clipped and that no
scrolling user interface should be provided to view the content
outside the clipping region.
scroll
This value indicates that the content is clipped and that if the
user agent uses a scrolling mechanism that is visible on the
screen (such as a scroll bar or a panner), that mechanism should
be displayed for a box whether or not any of its content is
clipped. This avoids any problem with scrollbars appearing and
disappearing in a dynamic environment. When this value is
specified and the target medium is 'print', overflowing content
may be printed.
auto
The behavior of the 'auto' value is user agent-dependent, but
should cause a scrolling mechanism to be provided for
overflowing boxes.
Even if 'overflow' is set to 'visible', content may be clipped to a
UA's document window by the native operating environment.
UAs must apply the 'overflow' property set on the root element to the
viewport. When the root element is an HTML "HTML" element or an XHTML
"html" element, and that element has an HTML "BODY" element or an XHTML
"body" element as a child, user agents must instead apply the
'overflow' property from the first such child element to the viewport,
if the value on the root element is 'visible'. The 'visible' value when
used for the viewport must be interpreted as 'auto'. The element from
which the value is propagated must have a used value for 'overflow' of
'visible'.
In the case of a scrollbar being placed on an edge of the element's
box, it should be inserted between the inner border edge and the outer
padding edge. Any space taken up by the scrollbars should be taken out
of (subtracted from the dimensions of) the containing block formed by
the element with the scrollbars.
Example(s):
Consider the following example of a block quotation (
) that
is too big for its containing block (established by a
). Here is
the source:
I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.
- Groucho Marx
Here is the style sheet controlling the sizes and style of the
generated boxes:
div { width : 100px; height: 100px;
border: thin solid red;
}
blockquote { width : 125px; height : 100px;
margin-top: 50px; margin-left: 50px;
border: thin dashed black
}
cite { display: block;
text-align : right;
border: none
}
The initial value of 'overflow' is 'visible', so the
would
be formatted without clipping, something like this:
Rendered overflow [D]
Setting 'overflow' to 'hidden' for the
, on the other hand, causes
the
to be clipped by the containing block:
Clipped overflow [D]
A value of 'scroll' would tell UAs that support a visible scrolling
mechanism to display one so that users could access the clipped
content.
Finally, consider this case where an absolutely positioned element is
mixed with an overflow parent.
Style sheet:
container { position: relative; border: solid; }
scroller { overflow: scroll; height: 5em; margin: 5em; }
satellite { position: absolute; top: 0; }
body { height: 10em; }
Document fragment:
In this example, the "scroller" element will not scroll the "satellite"
element, because the latter's containing block is outside the element
whose overflow is being clipped and scrolled.
11.1.2 Clipping: the 'clip' property
A clipping region defines what portion of an element's border box is
visible. By default, the element is not clipped. However, the clipping
region may be explicitly set with the 'clip' property.
'clip'
Value: | auto | inherit
Initial: auto
Applies to: absolutely positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: For rectangle values, a rectangle consisting of four
computed lengths; otherwise, as specified
The 'clip' property applies only to absolutely positioned elements.
Values have the following meanings:
auto
The element does not clip.
In CSS 2.1, the only valid value is: rect(,
, , ) where and specify
offsets from the top border edge of the box, and , and
specify offsets from the left border edge of the box in
left-to-right text and from the right border edge of the box in
right-to-left text. Authors should separate offset values with
commas. User agents must support separation with commas, but may
also support separation without commas (but not a combination),
because a previous revision of this specification was ambiguous
in this respect.
, , , and may either have a
value or 'auto'. Negative lengths are permitted. The value
'auto' means that a given edge of the clipping region will be
the same as the edge of the element's generated border box
(i.e., 'auto' means the same as '0' for and (in
left-to-right text, in right-to-left text), the same as
the computed value of the height plus the sum of vertical
padding and border widths for , and the same as the
computed value of the width plus the sum of the horizontal
padding and border widths for (in left-to-right text,
in right-to-left text), such that four 'auto' values
result in the clipping region being the same as the element's
border box).
When coordinates are rounded to pixel coordinates, care should
be taken that no pixels remain visible when and
have the same value (or and have the same value),
and conversely that no pixels within the element's border box
remain hidden when these values are 'auto'.
An element's clipping region clips out any aspect of the element (e.g.
content, children, background, borders, text decoration, outline and
visible scrolling mechanism -- if any) that is outside the clipping
region. Content that has been clipped does not cause overflow.
The element's ancestors may also clip portions of their content (e.g.
via their own 'clip' property and/or if their 'overflow' property is
not 'visible'); what is rendered is the cumulative intersection.
If the clipping region exceeds the bounds of the UA's document window,
content may be clipped to that window by the native operating
environment.
Example(s):
The following two rules:
p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }
will create the rectangular clipping regions delimited by the dashed
lines in the following illustrations:
Two clipping regions [D]
Note. In CSS 2.1, all clipping regions are rectangular. We anticipate
future extensions to permit non-rectangular clipping. Future updates
may also reintroduce a syntax for offsetting shapes from each edge
instead of offsetting from a point.
11.2 Visibility: the 'visibility' property
'visibility'
Value: visible | hidden | collapse | inherit
Initial: visible
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'visibility' property specifies whether the boxes generated by an
element are rendered. Invisible boxes still affect layout (set the
'display' property to 'none' to suppress box generation altogether).
Values have the following meanings:
visible
The generated box is visible.
hidden
The generated box is invisible (fully transparent, nothing is
drawn), but still affects layout. Furthermore, descendents of
the element will be visible if they have 'visibility: visible'.
collapse
Please consult the section on dynamic row and column effects in
tables. If used on elements other than rows, row groups,
columns, or column groups, 'collapse' has the same meaning as
'hidden'.
This property may be used in conjunction with scripts to create dynamic
effects.
In the following example, pressing either form button invokes an
author-defined script function that causes the corresponding box to
become visible and the other to be hidden. Since these boxes have the
same size and position, the effect is that one replaces the other. (The
script code is in a hypothetical script language. It may or may not
have any effect in a CSS-capable UA.)
Dynamic visibility example
Choose a suspect:
Name: Al Capone
Residence: Chicago
Name: Lucky Luciano
Residence: New York
__________________________________________________________________
__________________________________________________________________
12 Generated content, automatic numbering, and lists
Contents
* 12.1 The :before and :after pseudo-elements
* 12.2 The 'content' property
* 12.3 Quotation marks
+ 12.3.1 Specifying quotes with the 'quotes' property
+ 12.3.2 Inserting quotes with the 'content' property
* 12.4 Automatic counters and numbering
+ 12.4.1 Nested counters and scope
+ 12.4.2 Counter styles
+ 12.4.3 Counters in elements with 'display: none'
* 12.5 Lists
+ 12.5.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
In some cases, authors may want user agents to render content that does
not come from the document tree. One familiar example of this is a
numbered list; the author does not want to list the numbers explicitly,
he or she wants the user agent to generate them automatically.
Similarly, authors may want the user agent to insert the word "Figure"
before the caption of a figure, or "Chapter 7" before the seventh
chapter title. For audio or braille in particular, user agents should
be able to insert these strings.
In CSS 2.1, content may be generated by two mechanisms:
* The 'content' property, in conjunction with the :before and :after
pseudo-elements.
* Elements with a value of 'list-item' for the 'display' property.
12.1 The :before and :after pseudo-elements
Authors specify the style and location of generated content with the
:before and :after pseudo-elements. As their names indicate, the
:before and :after pseudo-elements specify the location of content
before and after an element's document tree content. The 'content'
property, in conjunction with these pseudo-elements, specifies what is
inserted.
Example(s):
For example, the following rule inserts the string "Note: " before the
content of every P element whose "class" attribute has the value
"note":
p.note:before { content: "Note: " }
The formatting objects (e.g., boxes) generated by an element include
generated content. So, for example, changing the above style sheet to:
p.note:before { content: "Note: " }
p.note { border: solid green }
would cause a solid green border to be rendered around the entire
paragraph, including the initial string.
The :before and :after pseudo-elements inherit any inheritable
properties from the element in the document tree to which they are
attached.
Example(s):
For example, the following rules insert an open quote mark before every
Q element. The color of the quote mark will be red, but the font will
be the same as the font of the rest of the Q element:
q:before {
content: open-quote;
color: red
}
In a :before or :after pseudo-element declaration, non-inherited
properties take their initial values.
Example(s):
So, for example, because the initial value of the 'display' property is
'inline', the quote in the previous example is inserted as an inline
box (i.e., on the same line as the element's initial text content). The
next example explicitly sets the 'display' property to 'block', so that
the inserted text becomes a block:
body:after {
content: "The End";
display: block;
margin-top: 2em;
text-align: center;
}
The :before and :after pseudo-elements elements interact with other
boxes, such as run-in boxes, as if they were real elements inserted
just inside their associated element.
Example(s):
For example, the following document fragment and style sheet:
Header
h2 { display: run-in; }
Text
p:before { display: block; content: 'Some'; }
...would render in exactly the same way as the following document
fragment and style sheet:
Header
h2 { display: run-in; }
Some Text
span { display: block }
Similarly, the following document fragment and style sheet:
...would render in exactly the same way as the following document
fragment and style sheet:
Header Thing
h2 { display: block; }
span { display: block; }
Text
Note. This specification does not fully define the interaction of
:before and :after with replaced elements (such as IMG in HTML). This
will be defined in more detail in a future specification.
12.2 The 'content' property
'content'
Value: normal | none | [ | | |
attr() | open-quote | close-quote | no-open-quote |
no-close-quote ]+ | inherit
Initial: normal
Applies to: :before and :after pseudo-elements
Inherited: no
Percentages: N/A
Media: all
Computed value: On elements, always computes to 'normal'. On :before
and :after, if 'normal' is specified, computes to 'none'. Otherwise,
for URI values, the absolute URI; for attr() values, the resulting
string; for other keywords, as specified.
This property is used with the :before and :after pseudo-elements to
generate content in a document. Values have the following meanings:
none
The pseudo-element is not generated.
normal
Computes to 'none' for the :before and :after pseudo-elements.
Text content (see the section on strings).
The value is a URI that designates an external resource (such as
an image). If the user agent cannot display the resource it must
either leave it out as if it were not specified or display some
indication that the resource cannot be displayed.
Counters may be specified with two different functions:
'counter()' or 'counters()'. The former has two forms:
'counter(name)' or 'counter(name, style)'. The generated text is
the value of the innermost counter of the given name in scope at
this pseudo-element; it is formatted in the indicated style
('decimal' by default). The latter function also has two forms:
'counters(name, string)' or 'counters(name, string, style)'. The
generated text is the value of all counters with the given name
in scope at this pseudo-element, from outermost to innermost
separated by the specified string. The counters are rendered in
the indicated style ('decimal' by default). See the section on
automatic counters and numbering for more information. The name
must not be 'none', 'inherit' or 'initial'. Such a name causes
the declaration to be ignored.
open-quote and close-quote
These values are replaced by the appropriate string from the
'quotes' property.
no-open-quote and no-close-quote
Introduces no content, but increments (decrements) the level of
nesting for quotes.
attr(X)
This function returns as a string the value of attribute X for
the subject of the selector. The string is not parsed by the CSS
processor. If the subject of the selector doesn't have an
attribute X, an empty string is returned. The case-sensitivity
of attribute names depends on the document language.
Note. In CSS 2.1, it is not possible to refer to attribute values for
other elements than the subject of the selector.
The 'display' property controls whether the content is placed in a
block or inline box.
Example(s):
The following rule causes the string "Chapter: " to be generated before
each H1 element:
H1:before {
content: "Chapter: ";
display: inline;
}
Authors may include newlines in the generated content by writing the
"\A" escape sequence in one of the strings after the 'content'
property. This inserted line break is still subject to the
'white-space' property. See "Strings" and "Characters and case" for
more information on the "\A" escape sequence.
Example(s):
h1:before {
display: block;
text-align: center;
white-space: pre;
content: "chapter\A hoofdstuk\A chapitre"
}
Generated content does not alter the document tree. In particular, it
is not fed back to the document language processor (e.g., for
reparsing).
12.3 Quotation marks
In CSS 2.1, authors may specify, in a style-sensitive and
context-dependent manner, how user agents should render quotation
marks. The 'quotes' property specifies pairs of quotation marks for
each level of embedded quotation. The 'content' property gives access
to those quotation marks and causes them to be inserted before and
after a quotation.
12.3.1 Specifying quotes with the 'quotes' property
'quotes'
Value: []+ | none | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies quotation marks for any number of embedded
quotations. Values have the following meanings:
none
The 'open-quote' and 'close-quote' values of the 'content'
property produce no quotation marks.
[]+
Values for the 'open-quote' and 'close-quote' values of the
'content' property are taken from this list of pairs of
quotation marks (opening and closing). The first (leftmost) pair
represents the outermost level of quotation, the second pair the
first level of embedding, etc. The user agent must apply the
appropriate pair of quotation marks according to the level of
embedding.
Example(s):
For example, applying the following style sheet:
/* Specify pairs of quotes for two levels in two languages */
q:lang(en) { quotes: '"' '"' "'" "'" }
q:lang(no) { quotes: "«" "»" '"' '"' }
/* Insert quotes before and after Q element content */
q:before { content: open-quote }
q:after { content: close-quote }
to the following HTML fragment:
Quotes
Quote me!
would allow a user agent to produce:
"Quote me!"
while this HTML fragment:
Quotes
Trøndere gråter når Vinsjan på kaia blir deklamert.
would produce:
«Trøndere gråter når "Vinsjan på kaia" blir deklamert.»
Note. While the quotation marks specified by 'quotes' in the previous
examples are conveniently located on computer keyboards, high quality
typesetting would require different ISO 10646 characters. The following
informative table lists some of the ISO 10646 quotation mark
characters:
Character Approximate rendering ISO 10646 code (hex) Description
" " 0022 QUOTATION MARK [the ASCII double quotation mark]
' ' 0027 APOSTROPHE [the ASCII single quotation mark]
< < 2039 SINGLE LEFT-POINTING ANGLE QUOTATION MARK
> > 203A SINGLE RIGHT-POINTING ANGLE QUOTATION MARK
« « 00AB LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
» » 00BB RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
` ` 2018 LEFT SINGLE QUOTATION MARK [single high-6]
' ' 2019 RIGHT SINGLE QUOTATION MARK [single high-9]
" `` 201C LEFT DOUBLE QUOTATION MARK [double high-6]
" '' 201D RIGHT DOUBLE QUOTATION MARK [double high-9]
" ,, 201E DOUBLE LOW-9 QUOTATION MARK [double low-9]
12.3.2 Inserting quotes with the 'content' property
Quotation marks are inserted in appropriate places in a document with
the 'open-quote' and 'close-quote' values of the 'content' property.
Each occurrence of 'open-quote' or 'close-quote' is replaced by one of
the strings from the value of 'quotes', based on the depth of nesting.
'Open-quote' refers to the first of a pair of quotes, 'close-quote'
refers to the second. Which pair of quotes is used depends on the
nesting level of quotes: the number of occurrences of 'open-quote' in
all generated text before the current occurrence, minus the number of
occurrences of 'close-quote'. If the depth is 0, the first pair is
used, if the depth is 1, the second pair is used, etc. If the depth is
greater than the number of pairs, the last pair is repeated. A
'close-quote' or 'no-close-quote' that would make the depth negative is
in error and is ignored (at rendering time): the depth stays at 0 and
no quote mark is rendered (although the rest of the 'content'
property's value is still inserted).
Note. The quoting depth is independent of the nesting of the source
document or the formatting structure.
Some typographic styles require open quotation marks to be repeated
before every paragraph of a quote spanning several paragraphs, but only
the last paragraph ends with a closing quotation mark. In CSS, this can
be achieved by inserting "phantom" closing quotes. The keyword
'no-close-quote' decrements the quoting level, but does not insert a
quotation mark.
Example(s):
The following style sheet puts opening quotation marks on every
paragraph in a BLOCKQUOTE, and inserts a single closing quote at the
end:
blockquote p:before { content: open-quote }
blockquote p:after { content: no-close-quote }
blockquote p.last:after { content: close-quote }
This relies on the last paragraph being marked with a class "last".
For symmetry, there is also a 'no-open-quote' keyword, which inserts
nothing, but increments the quotation depth by one.
12.4 Automatic counters and numbering
Automatic numbering in CSS 2.1 is controlled with two properties,
'counter-increment' and 'counter-reset'. The counters defined by these
properties are used with the counter() and counters() functions of the
the 'content' property.
'counter-reset'
Value: [ ? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
'counter-increment'
Value: [ ? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
The 'counter-increment' property accepts one or more names of counters
(identifiers), each one optionally followed by an integer. The integer
indicates by how much the counter is incremented for every occurrence
of the element. The default increment is 1. Zero and negative integers
are allowed.
The 'counter-reset' property also contains a list of one or more names
of counters, each one optionally followed by an integer. The integer
gives the value that the counter is set to on each occurrence of the
element. The default is 0.
The keywords 'none', 'inherit' and 'initial' must not be used as
counter names. A value of 'none' on its own means no counters are
reset, resp. incremented. 'Inherit' on its own has its usual meaning
(see 6.2.1). 'Initial' is reserved for future use.
Example(s):
This example shows a way to number chapters and sections with "Chapter
1", "1.1", "1.2", etc.
BODY {
counter-reset: chapter; /* Create a chapter counter scope */
}
H1:before {
content: "Chapter " counter(chapter) ". ";
counter-increment: chapter; /* Add 1 to chapter */
}
H1 {
counter-reset: section; /* Set section to 0 */
}
H2:before {
content: counter(chapter) "." counter(section) " ";
counter-increment: section;
}
If an element increments/resets a counter and also uses it (in the
'content' property of its :before or :after pseudo-element), the
counter is used after being incremented/reset.
If an element both resets and increments a counter, the counter is
reset first and then incremented.
If the same counter is specified more than once in the value of the
'counter-reset' and 'counter-increment' properties, each
reset/increment of the counter is processed in the order specified.
Example(s):
The following example will reset the 'section' counter to 0:
H1 { counter-reset: section 2 section }
The following example will increment the 'chapter' counter by 3:
H1 { counter-increment: chapter chapter 2 }
The 'counter-reset' property follows the cascading rules. Thus, due to
cascading, the following style sheet:
H1 { counter-reset: section -1 }
H1 { counter-reset: imagenum 99 }
will only reset 'imagenum'. To reset both counters, they have to be
specified together:
H1 { counter-reset: section -1 imagenum 99 }
12.4.1 Nested counters and scope
Counters are "self-nesting", in the sense that resetting a counter in a
descendant element or pseudo-element automatically creates a new
instance of the counter. This is important for situations like lists in
HTML, where elements can be nested inside themselves to arbitrary
depth. It would be impossible to define uniquely named counters for
each level.
Example(s):
Thus, the following suffices to number nested list items. The result is
very similar to that of setting 'display:list-item' and 'list-style:
inside' on the LI element:
OL { counter-reset: item }
LI { display: block }
LI:before { content: counter(item) ". "; counter-increment: item }
The scope of a counter starts at the first element in the document that
has a 'counter-reset' for that counter and includes the element's
descendants and its following siblings with their descendants. However,
it does not include any elements in the scope of a counter with the
same name created by a 'counter-reset' on a later sibling of the
element or by a later 'counter-reset' on the same element.
If 'counter-increment' or 'content' on an element or pseudo-element
refers to a counter that is not in the scope of any 'counter-reset',
implementations should behave as though a 'counter-reset' had reset the
counter to 0 on that element or pseudo-element.
In the example above, an OL will create a counter, and all children of
the OL will refer to that counter.
If we denote by item[n] the n^th instance of the "item" counter, and by
"{" and "}" the beginning and end of a scope, then the following HTML
fragment will use the indicated counters. (We assume the style sheet as
given in the example above).
item
item
item
item
item
item
item
item
item
item
item
item
Example(s):
Another example, showing how scope works when counters are used on
elements that are not nested, is the following. This shows how the
style rules given above to number chapters and sections would apply to
the markup given.
About CSS
CSS 2
CSS 2.1
Style
The 'counters()' function generates a string composed of all of the
counters with the same name that are in scope, separated by a given
string.
Example(s):
The following style sheet numbers nested list items as "1", "1.1",
"1.1.1", etc.
OL { counter-reset: item }
LI { display: block }
LI:before { content: counters(item, ".") " "; counter-increment: item }
12.4.2 Counter styles
By default, counters are formatted with decimal numbers, but all the
styles available for the 'list-style-type' property are also available
for counters. The notation is:
counter(name)
for the default style, or:
counter(name, <'list-style-type'>)
All the styles are allowed, including 'disc', 'circle', 'square', and
'none'.
Example(s):
H1:before { content: counter(chno, upper-latin) ". " }
H2:before { content: counter(section, upper-roman) " - " }
BLOCKQUOTE:after { content: " [" counter(bq, lower-greek) "]" }
DIV.note:before { content: counter(notecntr, disc) " " }
P:before { content: counter(p, none) }
12.4.3 Counters in elements with 'display: none'
An element that is not displayed ('display' set to 'none') cannot
increment or reset a counter.
Example(s):
For example, with the following style sheet, H2s with class "secret" do
not increment 'count2'.
H2.secret {counter-increment: count2; display: none}
Pseudo-elements that are not generated also cannot increment or reset a
counter.
Example(s):
For example, the following does not increment 'heading':
h1::before {
content: normal;
counter-increment: heading;
}
Elements with 'visibility' set to 'hidden', on the other hand, do
increment counters.
12.5 Lists
CSS 2.1 offers basic visual formatting of lists. An element with
'display: list-item' generates a principal box for the element's
content and an optional marker box as a visual indication that the
element is a list item.
The list properties describe basic visual formatting of lists: they
allow style sheets to specify the marker type (image, glyph, or
number), and the marker position with respect to the principal box
(outside it or within it before content). They do not allow authors to
specify distinct style (colors, fonts, alignment, etc.) for the list
marker or adjust its position with respect to the principal box; these
may be derived from the principal box.
The background properties apply to the principal box only; an 'outside'
marker box is transparent.
12.5.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
'list-style-type'
Value: disc | circle | square | decimal | decimal-leading-zero |
lower-roman | upper-roman | lower-greek | lower-latin | upper-latin |
armenian | georgian | lower-alpha | upper-alpha | none | inherit
Initial: disc
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies appearance of the list item marker if
'list-style-image' has the value 'none' or if the image pointed to by
the URI cannot be displayed. The value 'none' specifies no marker,
otherwise there are three types of marker: glyphs, numbering systems,
and alphabetic systems.
Glyphs are specified with disc, circle, and square. Their exact
rendering depends on the user agent.
Numbering systems are specified with:
decimal
Decimal numbers, beginning with 1.
decimal-leading-zero
Decimal numbers padded by initial zeros (e.g., 01, 02, 03, ...,
98, 99).
lower-roman
Lowercase roman numerals (i, ii, iii, iv, v, etc.).
upper-roman
Uppercase roman numerals (I, II, III, IV, V, etc.).
georgian
Traditional Georgian numbering (an, ban, gan, ..., he, tan, in,
in-an, ...).
armenian
Traditional Armenian numbering.
Alphabetic systems are specified with:
lower-latin or lower-alpha
Lowercase ascii letters (a, b, c, ... z).
upper-latin or upper-alpha
Uppercase ascii letters (A, B, C, ... Z).
lower-greek
Lowercase classical Greek alpha, beta, gamma, ... (a, b, g, ...)
This specification does not define how alphabetic systems wrap at the
end of the alphabet. For instance, after 26 list items, 'lower-latin'
rendering is undefined. Therefore, for long lists, we recommend that
authors specify true numbers.
For example, the following HTML document:
Lowercase latin numbering
This is the first item.
This is the second item.
This is the third item.
might produce something like this:
i This is the first item.
ii This is the second item.
iii This is the third item.
The list marker alignment (here, right justified) depends on the user
agent.
'list-style-image'
Value: | none | inherit
Initial: none
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: absolute URI or 'none'
This property sets the image that will be used as the list item marker.
When the image is available, it will replace the marker set with the
'list-style-type' marker.
The size of the image is calculated from the following rules:
1. If the image has an intrinsic width or height, then that intrinsic
width/height becomes the image's used width/height.
2. If the image's intrinsic width or height is given as a percentage,
then that percentage is resolved against 1em.
3. If the image has no intrinsic ratio and a ratio cannot be
calculated from its width and height, then its intrinsic ratio is
assumed to be 1:1.
4. If the image has a width but no height, its height is calculated
from the intrinsic ratio.
5. If the image's height cannot be resolved from the rules above, then
the image's height is assumed to be 1em.
6. If the image has no intrinsic width, then its width is calculated
from the resolved height and the intrinsic ratio.
Example(s):
The following example sets the marker at the beginning of each list
item to be the image "ellipse.png".
ul { list-style-image: url("http://png.com/ellipse.png") }
'list-style-position'
Value: inside | outside | inherit
Initial: outside
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the marker box in the principal
block box. Values have the following meanings:
outside
The marker box is outside the principal block box. CSS 2.1 does
not specify the precise location of the marker box, but does
require that for list items whose 'direction' property is 'ltr'
the marker box be on the left side of the content and for
elements whose 'direction' property is 'rtl' the marker box be
on the right side of the content. 'overflow' on the element does
not clip the marker box. The marker box is fixed with respect to
the principal block box's border and does not scroll with the
principal block box's content. The size or contents of the
marker box may affect the height of the principal block box
and/or the height of its first line box, and in some cases may
cause the creation of a new line box. Note: This interaction may
be more precisely defined in a future level of CSS.
inside
The marker box is the first inline box in the principal block
box, after which the element's content flows. CSS 2.1 does not
specify the precise location of the marker box.
For example:
Comparison of inside/outside position
first list item comes first
second list item comes second
first list item comes first
second list item comes second
The above example may be formatted as:
Difference between inside and outside list style position [D]
In right-to-left text, the markers would have been on the right side of
the box.
'list-style'
Value: [ <'list-style-type'> || <'list-style-position'> ||
<'list-style-image'> ] | inherit
Initial: see individual properties
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'list-style' property is a shorthand notation for setting the three
properties 'list-style-type', 'list-style-image', and
'list-style-position' at the same place in the style sheet.
Example(s):
ul { list-style: upper-roman inside } /* Any "ul" element */
ul > li > ul { list-style: circle outside } /* Any "ul" child
of an "li" child
of a "ul" element */
Although authors may specify 'list-style' information directly on list
item elements (e.g., "li" in HTML), they should do so with care. The
following rules look similar, but the first declares a descendant
selector and the second a (more specific) child selector.
ol.alpha li { list-style: lower-alpha } /* Any "li" descendant of an "ol" */
ol.alpha > li { list-style: lower-alpha } /* Any "li" child of an "ol" */
Authors who use only the descendant selector may not achieve the
results they expect. Consider the following rules:
WARNING: Unexpected results due to cascade
level 1
level 2
The desired rendering would have level 1 list items with 'lower-alpha'
labels and level 2 items with 'disc' labels. However, the cascading
order will cause the first style rule (which includes specific class
information) to mask the second. The following rules solve the problem
by employing a child selector instead:
ol.alpha > li { list-style: lower-alpha }
ul li { list-style: disc }
Another solution would be to specify 'list-style' information only on
the list type elements:
ol.alpha { list-style: lower-alpha }
ul { list-style: disc }
Inheritance will transfer the 'list-style' values from OL and UL
elements to LI elements. This is the recommended way to specify list
style information.
Example(s):
A URI value may be combined with any other value, as in:
ul { list-style: url("http://png.com/ellipse.png") disc }
In the example above, the 'disc' will be used when the image is
unavailable.
A value of 'none' within the 'list-style' property sets whichever of
'list-style-type' and 'list-style-image' are not otherwise specified to
'none'. However, if both are otherwise specified, the declaration is in
error (and thus ignored).
Example(s):
For example, a value of 'none' for the 'list-style' property sets both
'list-style-type' and 'list-style-image' to 'none':
ul { list-style: none }
The result is that no list-item marker is displayed.
__________________________________________________________________
__________________________________________________________________
13 Paged media
Contents
* 13.1 Introduction to paged media
* 13.2 Page boxes: the @page rule
+ 13.2.1 Page margins
+ 13.2.2 Page selectors: selecting left, right, and first pages
+ 13.2.3 Content outside the page box
* 13.3 Page breaks
+ 13.3.1 Page break properties: 'page-break-before',
'page-break-after', 'page-break-inside'
+ 13.3.2 Breaks inside elements: 'orphans', 'widows'
+ 13.3.3 Allowed page breaks
+ 13.3.4 Forced page breaks
+ 13.3.5 "Best" page breaks
* 13.4 Cascading in the page context
13.1 Introduction to paged media
Paged media (e.g., paper, transparencies, pages that are displayed on
computer screens, etc.) differ from continuous media in that the
content of the document is split into one or more discrete pages. To
handle pages, CSS 2.1 describes how page margins are set on page boxes,
and how page breaks are declared.
The user agent is responsible for transferring the page boxes of a
document onto the real sheets where the document will ultimately be
rendered (paper, transparency, screen, etc.). There is often a 1-to-1
relationship between a page box and a sheet, but this is not always the
case. Transfer possibilities include:
* Transferring one page box to one sheet (e.g., single-sided
printing).
* Transferring two page boxes to both sides of the same sheet (e.g.,
double-sided printing).
* Transferring N (small) page boxes to one sheet (called "n-up").
* Transferring one (large) page box to N x M sheets (called
"tiling").
* Creating signatures. A signature is a group of pages printed on a
sheet, which, when folded and trimmed like a book, appear in their
proper sequence.
* Printing one document to several output trays.
* Outputting to a file.
13.2 Page boxes: the @page rule
The page box is a rectangular region that contains two areas:
* The page area. The page area includes the boxes laid out on that
page. The edges of the first page area establish the rectangle that
is the initial containing block of the document. The canvas
background is painted within and covers the page area.
* The margin area, which surrounds the page area. The page margin
area is transparent.
The size of a page box cannot be specified in CSS 2.1.
Authors can specify the margins of a page box inside an @page rule. An
@page rule consists of the keyword "@page", followed by an optional
page selector, followed by a block of declarations. The declarations in
an @page rule are said to be in the page context.
The page selector specifies for which pages the declarations apply. In
CSS 2.1, page selectors may designate the first page, all left pages,
or all right pages
13.2.1 Page margins
In CSS 2.1, only the margin properties ('margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin') apply within the page
context. The following diagram shows the relationships between the
sheet, page box, and page margins:
Illustration of sheet, page box, margin, and page area. [D]
Example(s):
Here is a simple example which sets all page margins on all pages:
@page {
margin: 3cm;
}
The page context has no notion of fonts, so 'em' and 'ex' units are not
allowed. Percentage values on the margin properties are relative to the
dimensions of the page box; for left and right margins, they refer to
the width of the page box while for top and bottom margins, they refer
to the height of the page box. All other units associated with the
respective CSS 2.1 properties are allowed.
Due to negative margin values (either on the page box or on elements)
or absolute positioning content may end up outside the page box, but
this content may be "cut" -- by the user agent, the printer, or
ultimately, the paper cutter.
13.2.2 Page selectors: selecting left, right, and first pages
When printing double-sided documents, the page boxes on left and right
pages may be different. This can be expressed through two CSS
pseudo-classes that may be used in page selectors.
All pages are automatically classified by user agents into either the
:left or :right pseudo-class.
Example(s):
@page :left {
margin-left: 4cm;
margin-right: 3cm;
}
@page :right {
margin-left: 3cm;
margin-right: 4cm;
}
If different declarations have been given for left and right pages, the
user agent must honor these declarations even if the user agent does
not transfer the page boxes to left and right sheets (e.g., a printer
that only prints single-sided).
Authors may also specify style for the first page of a document with
the :first pseudo-class:
Example(s):
@page { margin: 2cm } /* All margins set to 2cm */
@page :first {
margin-top: 10cm /* Top margin on first page 10cm */
}
Properties specified in a :left or :right @page rule override those
specified in an @page rule that has no pseudo-class specified.
Properties specified in a :first @page rule override those specified in
:left or :right @page rules.
Margin declarations on left, right, and first pages may result in
different page area widths. To simplify implementations, user agents
may use a single page area width on left, right, and first pages. In
this case, the page area width of the first page should be used.
13.2.3 Content outside the page box
When formatting content in the page model, some content may end up
outside the current page box. For example, an element whose
'white-space' property has the value 'pre' may generate a box that is
wider than the page box. As another example, when boxes are positioned
absolutely or relatively, they may end up in "inconvenient" locations.
For example, images may be placed on the edge of the page box or
100,000 meters below the page box.
The exact formatting of such elements lies outside the scope of this
specification. However, we recommend that authors and user agents
observe the following general principles concerning content outside the
page box:
* Content should be allowed slightly beyond the page box to allow
pages to "bleed".
* User agents should avoid generating a large number of empty page
boxes to honor the positioning of elements (e.g., you don't want to
print 100 blank pages).
* Authors should not position elements in inconvenient locations just
to avoid rendering them.
* User agents may handle boxes positioned outside the page box in
several ways, including discarding them or creating page boxes for
them at the end of the document.
13.3 Page breaks
This section describes page breaks in CSS 2.1. Five properties indicate
where the user agent may or should break pages, and on what page (left
or right) the subsequent content should resume. Each page break ends
layout in the current page box and causes remaining pieces of the
document tree to be laid out in a new page box.
13.3.1 Page break properties: 'page-break-before', 'page-break-after',
'page-break-inside'
'page-break-before'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements (but see text)
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-after'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements (but see text)
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-inside'
Value: avoid | auto | inherit
Initial: auto
Applies to: block-level elements (but see text)
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
Values for these properties have the following meanings:
auto
Neither force nor forbid a page break before (after, inside) the
generated box.
always
Always force a page break before (after) the generated box.
avoid
Avoid a page break before (after, inside) the generated box.
left
Force one or two page breaks before (after) the generated box so
that the next page is formatted as a left page.
right
Force one or two page breaks before (after) the generated box so
that the next page is formatted as a right page.
Whether the first page of a document is :left or :right depends on the
major writing direction of the document. A conforming user agent may
interpret the values 'left' and 'right' as 'always'.
A potential page break location is typically under the influence of the
parent element's 'page-break-inside' property, the 'page-break-after'
property of the preceding element, and the 'page-break-before' property
of the following element. When these properties have values other than
'auto', the values 'always', 'left', and 'right' take precedence over
'avoid'.
User Agents must apply these properties to block-level elements in the
normal flow of the root element. User agents may also apply these
properties to other elements, e.g., 'table-row' elements.
When a page break splits a box, the box's margins, borders, and padding
have no visual effect where the split occurs.
13.3.2 Breaks inside elements: 'orphans', 'widows'
'orphans'
Value: | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
'widows'
Value: | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
The 'orphans' property specifies the minimum number of lines in a block
element that must be left at the bottom of a page. The 'widows'
property specifies the minimum number of lines in a block element that
must be left at the top of a page. Examples of how they are used to
control page breaks are given below.
Only positive values are allowed.
For information about paragraph formatting, please consult the section
on line boxes.
13.3.3 Allowed page breaks
In the normal flow, page breaks can occur at the following places:
1. In the vertical margin between block boxes. When an unforced page
break occurs here, the used values of the relevant 'margin-top' and
'margin-bottom' properties are set to '0'. When a forced page break
occurs here, the used value of the relevant 'margin-bottom'
property is set to '0'; the relevant 'margin-top' used value may
either be set to '0' or retained.
2. Between line boxes inside a block box.
Note: It is expected that CSS3 will specify that the relevant
'margin-top' applies (i.e., is not set to '0') after a forced page
break.
These breaks are subject to the following rules:
* Rule A: Breaking at (1) is allowed only if the 'page-break-after'
and 'page-break-before' properties of all the elements generating
boxes that meet at this margin allow it, which is when at least one
of them has the value 'always', 'left', or 'right', or when all of
them are 'auto'.
* Rule B: However, if all of them are 'auto' and a common ancestor of
all the elements has a 'page-break-inside' value of 'avoid', then
breaking here is not allowed.
* Rule C: Breaking at (2) is allowed only if the number of line boxes
between the break and the start of the enclosing block box is the
value of 'orphans' or more, and the number of line boxes between
the break and the end of the box is the value of 'widows' or more.
* Rule D: In addition, breaking at (2) is allowed only if the
'page-break-inside' property of the element and all its ancestors
is 'auto'.
If the above doesn't provide enough break points to keep content from
overflowing the page boxes, then rules A, B and D are dropped in order
to find additional breakpoints.
If that still does not lead to sufficient break points, rule C is
dropped as well, to find still more break points.
13.3.4 Forced page breaks
A page break must occur at (1) if, among the 'page-break-after' and
'page-break-before' properties of all the elements generating boxes
that meet at this margin, there is at least one with the value
'always', 'left', or 'right'.
13.3.5 "Best" page breaks
CSS 2.1 does not define which of a set of allowed page breaks must be
used; CSS 2.1 does not forbid a user agent from breaking at every
possible break point, or not to break at all. But CSS 2.1 does
recommend that user agents observe the following heuristics (while
recognizing that they are sometimes contradictory):
* Break as few times as possible.
* Make all pages that don't end with a forced break appear to have
about the same height.
* Avoid breaking inside a replaced element.
Example(s):
Suppose, for example, that the style sheet contains 'orphans: 4',
'widows: 2', and there are 20 lines (line boxes) available at the
bottom of the current page:
* If a paragraph at the end of the current page contains 20 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 21 or 22 lines, the second part of the
paragraph must not violate the 'widows' constraint, and so the
second part must contain exactly two lines
* If the paragraph contains 23 lines or more, the first part should
contain 20 lines and the second part the remaining lines.
Now suppose that 'orphans' is '10', 'widows' is '20', and there are 8
lines available at the bottom of the current page:
* If a paragraph at the end of the current page contains 8 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 9 lines or more, it cannot be split (that
would violate the orphan constraint), so it should move as a block
to the next page.
13.4 Cascading in the page context
Declarations in the page context obey the cascade just like normal CSS
declarations.
Example(s):
Consider the following example:
@page {
margin-left: 3cm;
}
@page :left {
margin-left: 4cm;
}
Due to the higher specificity of the pseudo-class selector, the left
margin on left pages will be '4cm' and all other pages (i.e., the right
pages) will have a left margin of '3cm'.
__________________________________________________________________
__________________________________________________________________
14 Colors and Backgrounds
Contents
* 14.1 Foreground color: the 'color' property
* 14.2 The background
+ 14.2.1 Background properties: 'background-color',
'background-image', 'background-repeat',
'background-attachment', 'background-position', and
'background'
* 14.3 Gamma correction
CSS properties allow authors to specify the foreground color and
background of an element. Backgrounds may be colors or images.
Background properties allow authors to position a background image,
repeat it, and declare whether it should be fixed with respect to the
viewport or scrolled along with the document.
See the section on color units for the syntax of valid color values.
14.1 Foreground color: the 'color' property
'color'
Value: | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property describes the foreground color of an element's text
content. There are different ways to specify red:
Example(s):
em { color: red } /* predefined color name */
em { color: rgb(255,0,0) } /* RGB range 0-255 */
14.2 The background
Authors may specify the background of an element (i.e., its rendering
surface) as either a color or an image. In terms of the box model,
"background" refers to the background of the content, padding and
border areas. Border colors and styles are set with the border
properties. Margins are always transparent.
Background properties are not inherited, but the parent box's
background will shine through by default because of the initial
'transparent' value on 'background-color'.
The background of the root element becomes the background of the canvas
and covers the entire canvas, anchored (for 'background-position') at
the same point as it would be if it was painted only for the root
element itself. The root element does not paint this background again.
For HTML documents, however, we recommend that authors specify the
background for the BODY element rather than the HTML element. For
documents whose root element is an HTML "HTML" element or an XHTML
"html" element that has computed values of 'transparent' for
'background-color' and 'none' for 'background-image', user agents must
instead use the computed value of the background properties from that
element's first HTML "BODY" element or XHTML "body" element child when
painting backgrounds for the canvas, and must not paint a background
for that child element. Such backgrounds must also be anchored at the
same point as they would be if they were painted only for the root
element.
According to these rules, the canvas underlying the following HTML
document will have a "marble" background:
Setting the canvas background
My background is marble.
Note that the rule for the BODY element will work even though the BODY
tag has been omitted in the HTML source since the HTML parser will
infer the missing tag.
Backgrounds of elements that form a stacking context (see the 'z-index'
property) are painted at the bottom of the element's stacking context,
below anything in that stacking context.
14.2.1 Background properties: 'background-color', 'background-image',
'background-repeat', 'background-attachment', 'background-position', and
'background'
'background-color'
Value: | transparent | inherit
Initial: transparent
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property sets the background color of an element, either a
value or the keyword 'transparent', to make the underlying colors shine
through.
Example(s):
h1 { background-color: #F00 }
'background-image'
Value: | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute URI or none
This property sets the background image of an element. When setting a
background image, authors should also specify a background color that
will be used when the image is unavailable. When the image is
available, it is rendered on top of the background color. (Thus, the
color is visible in the transparent parts of the image).
Values for this property are either , to specify the image, or
'none', when no image is used.
Example(s):
body { background-image: url("marble.png") }
p { background-image: none }
Intrinsic dimensions expressed as percentages must be resolved relative
to the dimensions of the rectangle that establishes the coordinate
system for the 'background-position' property.
If the image has one of either an intrinsic width or an intrinsic
height and an intrinsic aspect ratio, then the missing dimension is
calculated from the given dimension and the ratio.
If the image has one of either an intrinsic width or an intrinsic
height and no intrinsic aspect ratio, then the missing dimension is
assumed to be the size of the rectangle that establishes the coordinate
system for the 'background-position' property.
If the image has no intrinsic dimensions and has an intrinsic ratio the
dimensions must be assumed to be the largest dimensions at that ratio
such that neither dimension exceeds the dimensions of the rectangle
that establishes the coordinate system for the 'background-position'
property.
If the image has no intrinsic ratio either, then the dimensions must be
assumed to be the rectangle that establishes the coordinate system for
the 'background-position' property.
'background-repeat'
Value: repeat | repeat-x | repeat-y | no-repeat | inherit
Initial: repeat
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether the
image is repeated (tiled), and how. All tiling covers the content,
padding and border areas of a box.
The tiling and positioning of the background-image on inline elements
is undefined in this specification. A future level of CSS may define
the tiling and positioning of the background-image on inline elements.
Values have the following meanings:
repeat
The image is repeated both horizontally and vertically.
repeat-x
The image is repeated horizontally only.
repeat-y
The image is repeated vertically only.
no-repeat
The image is not repeated: only one copy of the image is drawn.
Example(s):
body {
background: white url("pendant.png");
background-repeat: repeat-y;
background-position: center;
}
A centered background image, with copies repeated up and down the
padding and content areas. [D]
One copy of the background image is centered, and other copies are put
above and below it to make a vertical band behind the element.
'background-attachment'
Value: scroll | fixed | inherit
Initial: scroll
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether it
is fixed with regard to the viewport ('fixed') or scrolls along with
the containing block ('scroll').
Note that there is only one viewport per view. If an element has a
scrolling mechanism (see 'overflow'), a 'fixed' background doesn't move
with the element, and a 'scroll' background doesn't move with the
scrolling mechanism.
Even if the image is fixed, it is still only visible when it is in the
content, padding or border area of the element. Thus, unless the image
is tiled ('background-repeat: repeat'), it may be invisible.
Example(s):
This example creates an infinite vertical band that remains "glued" to
the viewport when the element is scrolled.
body {
background: red url("pendant.png");
background-repeat: repeat-y;
background-attachment: fixed;
}
User agents that do not support 'fixed' backgrounds (for example due to
limitations of the hardware platform) should ignore declarations with
the keyword 'fixed'. For example:
body {
background: white url(paper.png) scroll; /* for all UAs */
background: white url(ledger.png) fixed; /* for UAs that do fixed backgrounds
*/
}
See the section on conformance for details.
'background-position'
Value: [ [ | | left | center | right ] [
| | top | center | bottom ]? ] | [ [ left |
center | right ] || [ top | center | bottom ] ] | inherit
Initial: 0% 0%
Applies to: all elements
Inherited: no
Percentages: refer to the size of the box itself
Media: visual
Computed value: for the absolute value, otherwise a
percentage
If a background image has been specified, this property specifies its
initial position. If only one value is specified, the second value is
assumed to be 'center'. If at least one value is not a keyword, then
the first value represents the horizontal position and the second
represents the vertical position. Negative and
values are allowed.
A percentage X aligns the point X% across (for horizontal) or
down (for vertical) the image with the point X% across (for
horizontal) or down (for vertical) the element's padding box.
For example, with a value pair of '0% 0%',the upper left corner
of the image is aligned with the upper left corner of the
padding box. A value pair of '100% 100%' places the lower right
corner of the image in the lower right corner of the padding
box. With a value pair of '14% 84%', the point 14% across and
84% down the image is to be placed at the point 14% across and
84% down the padding box.
A length L aligns the top left corner of the image a distance L
to the right of (for horizontal) or below (for vertical) the top
left corner of the element's padding box. For example, with a
value pair of '2cm 1cm', the upper left corner of the image is
placed 2cm to the right and 1cm below the upper left corner of
the padding box.
top
Equivalent to '0%' for the vertical position.
right
Equivalent to '100%' for the horizontal position.
bottom
Equivalent to '100%' for the vertical position.
left
Equivalent to '0%' for the horizontal position.
center
Equivalent to '50%' for the horizontal position if it is not
otherwise given, or '50%' for the vertical position if it is.
Example(s):
body { background: url("banner.jpeg") right top } /* 100% 0% */
body { background: url("banner.jpeg") top center } /* 50% 0% */
body { background: url("banner.jpeg") center } /* 50% 50% */
body { background: url("banner.jpeg") bottom } /* 50% 100% */
The tiling and positioning of the background-image on inline elements
is undefined in this specification. A future level of CSS may define
the tiling and positioning of the background-image on inline elements.
If the background image is fixed within the viewport (see the
'background-attachment' property), the image is placed relative to the
viewport instead of the element's padding box. For example,
Example(s):
body {
background-image: url("logo.png");
background-attachment: fixed;
background-position: 100% 100%;
background-repeat: no-repeat;
}
In the example above, the (single) image is placed in the lower-right
corner of the viewport.
'background'
Value: [<'background-color'> || <'background-image'> ||
<'background-repeat'> || <'background-attachment'> ||
<'background-position'>] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: allowed on 'background-position'
Media: visual
Computed value: see individual properties
The 'background' property is a shorthand property for setting the
individual background properties (i.e., 'background-color',
'background-image', 'background-repeat', 'background-attachment' and
'background-position') at the same place in the style sheet.
Given a valid declaration, the 'background' property first sets all the
individual background properties to their initial values, then assigns
explicit values given in the declaration.
Example(s):
In the first rule of the following example, only a value for
'background-color' has been given and the other individual properties
are set to their initial value. In the second rule, all individual
properties have been specified.
BODY { background: red }
P { background: url("chess.png") gray 50% repeat fixed }
14.3 Gamma correction
For information about gamma issues, please consult the Gamma Tutorial
in the PNG specification ([PNG]).
Note. In the computation of gamma correction, UAs displaying on a CRT
may assume an ideal CRT and ignore any effects on apparent gamma caused
by dithering. That means the minimal handling they need to do on
current platforms is:
PC using MS-Windows
none
Unix using X11
none
Mac using QuickDraw
apply gamma 1.45 [ICC42] (ColorSync-savvy applications may
simply pass the sRGB ICC profile to ColorSync to perform correct
color correction)
SGI using X
apply the gamma value from /etc/config/system.glGammaVal (the
default value being 1.70; applications running on Irix 6.2 or
above may simply pass the sRGB ICC profile to the color
management system)
NeXT using NeXTStep
apply gamma 2.22
"Applying gamma" means that each of the three R, G and B must be
converted to R'=R^gamma, G'=G^gamma, B'=B^gamma, before being handed to
the OS.
This may be done rapidly by building a 256-element lookup table once
per browser invocation thus:
for i := 0 to 255 do
raw := i / 255.0;
corr := pow (raw, gamma);
table[i] := trunc (0.5 + corr * 255.0)
end
which then avoids any need to do transcendental math per color
attribute, far less per pixel.
__________________________________________________________________
__________________________________________________________________
15 Fonts
Contents
* 15.1 Introduction
* 15.2 Font matching algorithm
* 15.3 Font family: the 'font-family' property
+ 15.3.1 Generic font families
o 15.3.1.1 serif
o 15.3.1.2 sans-serif
o 15.3.1.3 cursive
o 15.3.1.4 fantasy
o 15.3.1.5 monospace
* 15.4 Font styling: the 'font-style' property
* 15.5 Small-caps: the 'font-variant' property
* 15.6 Font boldness: the 'font-weight' property
* 15.7 Font size: the 'font-size' property
* 15.8 Shorthand font property: the 'font' property
15.1 Introduction
Setting font properties will be among the most common uses of style
sheets. Unfortunately, there exists no well-defined and universally
accepted taxonomy for classifying fonts, and terms that apply to one
font family may not be appropriate for others. E.g. 'italic' is
commonly used to label slanted text, but slanted text may also be
labeled as being Oblique, Slanted, Incline, Cursive or Kursiv.
Therefore it is not a simple problem to map typical font selection
properties to a specific font.
15.2 Font matching algorithm
Because there is no accepted, universal taxonomy of font properties,
matching of properties to font faces must be done carefully. The
properties are matched in a well-defined order to insure that the
results of this matching process are as consistent as possible across
UAs (assuming that the same library of font faces is presented to each
of them).
1. The User Agent makes (or accesses) a database of relevant CSS 2.1
properties of all the fonts of which the UA is aware. If there are
two fonts with exactly the same properties, the user agent selects
one of them.
2. At a given element and for each character in that element, the UA
assembles the font properties applicable to that element. Using the
complete set of properties, the UA uses the 'font-family' property
to choose a tentative font family. The remaining properties are
tested against the family according to the matching criteria
described with each property. If there are matches for all the
remaining properties, then that is the matching font face for the
given element or character.
3. If there is no matching font face within the 'font-family' being
processed by step 2, and if there is a next alternative
'font-family' in the font set, then repeat step 2 with the next
alternative 'font-family'.
4. If there is a matching font face, but it doesn't contain a glyph
for the current character, and if there is a next alternative
'font-family' in the font sets, then repeat step 2 with the next
alternative 'font-family'.
5. If there is no font within the family selected in 2, then use a
UA-dependent default 'font-family' and repeat step 2, using the
best match that can be obtained within the default font. If a
particular character cannot be displayed using this font, then the
UA may use other means to determine a suitable font for that
character. The UA should map each character for which it has no
suitable font to a visible symbol chosen by the UA, preferably a
"missing character" glyph from one of the font faces available to
the UA.
(The above algorithm can be optimized to avoid having to revisit the
CSS 2.1 properties for each character.)
The per-property matching rules from (2) above are as follows:
1. 'font-style' is tried first. 'italic' will be satisfied if there is
either a face in the UA's font database labeled with the CSS
keyword 'italic' (preferred) or 'oblique'. Otherwise the values
must be matched exactly or font-style will fail.
2. 'font-variant' is tried next. 'small-caps' matches (1) a font
labeled as 'small-caps', (2) a font in which the small caps are
synthesized, or (3) a font where all lowercase letters are replaced
by upper case letters. A small-caps font may be synthesized by
electronically scaling uppercase letters from a normal font.
'normal' matches a font's normal (non-small-caps) variant. A font
cannot fail to have a normal variant. A font that is only available
as small-caps shall be selectable as either a 'normal' face or a
'small-caps' face.
3. 'font-weight' is matched next, it will never fail. (See
'font-weight' below.)
4. 'font-size' must be matched within a UA-dependent margin of
tolerance. (Typically, sizes for scalable fonts are rounded to the
nearest whole pixel, while the tolerance for bitmapped fonts could
be as large as 20%.) Further computations, e.g. by 'em' values in
other properties, are based on the computed value of 'font-size'.
15.3 Font family: the 'font-family' property
'font-family'
Value: [[ | ] [, |
]* ] | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The value is a prioritized list of font family names and/or generic
family names. Unlike most other CSS properties, values are separated by
a comma to indicate that they are alternatives:
body { font-family: Gill, Helvetica, sans-serif }
Although many fonts provide the "missing character" glyph, typically an
open box, as its name implies this should not be considered a match for
characters that cannot be found in the font. (It should, however, be
considered a match for U+FFFD, the "missing character" character's code
point).
There are two types of font family names:
The name of a font family of choice. In the last example, "Gill"
and "Helvetica" are font families.
In the example above, the last value is a generic family name.
The following generic families are defined:
+ 'serif' (e.g. Times)
+ 'sans-serif' (e.g. Helvetica)
+ 'cursive' (e.g. Zapf-Chancery)
+ 'fantasy' (e.g. Western)
+ 'monospace' (e.g. Courier)
Style sheet designers are encouraged to offer a generic font
family as a last alternative. Generic font family names are
keywords and must NOT be quoted.
If an unquoted font family name contains parentheses, brackets, and/or
braces, they must still be escaped per CSS grammar rules. Similarly,
quotation marks (both single and double), semicolons, exclamation
marks, commas, and leading slashes within unquoted font family names
must be escaped. Font names containing any such characters or white
space should be quoted:
body { font-family: "New Century Schoolbook", serif }
If quoting is omitted, any white space characters before and after the
font name are ignored and any sequence of white space characters inside
the font name is converted to a single space. Font family names that
happen to be the same as a keyword value (e.g. 'initial', 'inherit',
'default', 'serif', 'sans-serif', 'monospace', 'fantasy', and
'cursive') must be quoted to prevent confusion with the keywords with
the same names. UAs must not consider these keywords as matching the
'' type.
15.3.1 Generic font families
Generic font families are a fallback mechanism, a means of preserving
some of the style sheet author's intent in the worst case when none of
the specified fonts can be selected. For optimum typographic control,
particular named fonts should be used in style sheets.
All five generic font families are defined to exist in all CSS
implementations (they need not necessarily map to five distinct actual
fonts). User agents should provide reasonable default choices for the
generic font families, which express the characteristics of each family
as well as possible within the limits allowed by the underlying
technology.
User agents are encouraged to allow users to select alternative choices
for the generic fonts.
15.3.1.1 serif
Glyphs of serif fonts, as the term is used in CSS, have finishing
strokes, flared or tapering ends, or have actual serifed endings
(including slab serifs). Serif fonts are typically
proportionately-spaced. They often display a greater variation between
thick and thin strokes than fonts from the 'sans-serif' generic font
family. CSS uses the term 'serif' to apply to a font for any script,
although other names may be more familiar for particular scripts, such
as Mincho (Japanese), Sung or Song (Chinese), Totum or Kodig (Korean).
Any font that is so described may be used to represent the generic
'serif' family.
Examples of fonts that fit this description include:
Latin fonts Times New Roman, Bodoni, Garamond, Minion Web, ITC Stone
Serif, MS Georgia, Bitstream Cyberbit
Greek fonts Bitstream Cyberbit
Cyrillic fonts Adobe Minion Cyrillic, Excelcior Cyrillic Upright,
Monotype Albion 70, Bitstream Cyberbit, ER Bukinst
Hebrew fonts New Peninim, Raanana, Bitstream Cyberbit
Japanese fonts Ryumin Light-KL, Kyokasho ICA, Futo Min A101
Arabic fonts Bitstream Cyberbit
Cherokee fonts Lo Cicero Cherokee
15.3.1.2 sans-serif
Glyphs in sans-serif fonts, as the term is used in CSS, have stroke
endings that are plain -- without any flaring, cross stroke, or other
ornamentation. Sans-serif fonts are typically proportionately-spaced.
They often have little variation between thick and thin strokes,
compared to fonts from the 'serif' family. CSS uses the term
'sans-serif' to apply to a font for any script, although other names
may be more familiar for particular scripts, such as Gothic (Japanese),
Kai (Chinese), or Pathang (Korean). Any font that is so described may
be used to represent the generic 'sans-serif' family.
Examples of fonts that fit this description include:
Latin fonts MS Trebuchet, ITC Avant Garde Gothic, MS Arial, MS Verdana,
Univers, Futura, ITC Stone Sans, Gill Sans, Akzidenz Grotesk, Helvetica
Greek fonts Attika, Typiko New Era, MS Tahoma, Monotype Gill Sans 571,
Helvetica Greek
Cyrillic fonts Helvetica Cyrillic, ER Univers, Lucida Sans Unicode,
Bastion
Hebrew fonts Arial Hebrew, MS Tahoma
Japanese fonts Shin Go, Heisei Kaku Gothic W5
Arabic fonts MS Tahoma
15.3.1.3 cursive
Glyphs in cursive fonts, as the term is used in CSS, generally have
either joining strokes or other cursive characteristics beyond those of
italic typefaces. The glyphs are partially or completely connected, and
the result looks more like handwritten pen or brush writing than
printed letterwork. Fonts for some scripts, such as Arabic, are almost
always cursive. CSS uses the term 'cursive' to apply to a font for any
script, although other names such as Chancery, Brush, Swing and Script
are also used in font names.
Examples of fonts that fit this description include:
Latin fonts Caflisch Script, Adobe Poetica, Sanvito, Ex Ponto, Snell
Roundhand, Zapf-Chancery
Cyrillic fonts ER Architekt
Hebrew fonts Corsiva
Arabic fonts DecoType Naskh, Monotype Urdu 507
15.3.1.4 fantasy
Fantasy fonts, as used in CSS, are primarily decorative while still
containing representations of characters (as opposed to Pi or Picture
fonts, which do not represent characters). Examples include:
Latin fonts Alpha Geometrique, Critter, Cottonwood, FB Reactor, Studz
15.3.1.5 monospace
The sole criterion of a monospace font is that all glyphs have the same
fixed width. (This can make some scripts, such as Arabic, look most
peculiar.) The effect is similar to a manual typewriter, and is often
used to set samples of computer code.
Examples of fonts which fit this description include:
Latin fonts Courier, MS Courier New, Prestige, Everson Mono
Greek Fonts MS Courier New, Everson Mono
Cyrillic fonts ER Kurier, Everson Mono
Japanese fonts Osaka Monospaced
Cherokee fonts Everson Mono
15.4 Font styling: the 'font-style' property
'font-style'
Value: normal | italic | oblique | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'font-style' property selects between normal (sometimes referred to
as "roman" or "upright"), italic and oblique faces within a font
family.
A value of 'normal' selects a font that is classified as 'normal' in
the UA's font database, while 'oblique' selects a font that is labeled
'oblique'. A value of 'italic' selects a font that is labeled 'italic',
or, if that is not available, one labeled 'oblique'.
The font that is labeled 'oblique' in the UA's font database may
actually have been generated by electronically slanting a normal font.
Fonts with Oblique, Slanted or Incline in their names will typically be
labeled 'oblique' in the UA's font database. Fonts with Italic, Cursive
or Kursiv in their names will typically be labeled 'italic'.
h1, h2, h3 { font-style: italic }
h1 em { font-style: normal }
In the example above, emphasized text within 'H1' will appear in a
normal face.
15.5 Small-caps: the 'font-variant' property
'font-variant'
Value: normal | small-caps | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
Another type of variation within a font family is the small-caps. In a
small-caps font the lower case letters look similar to the uppercase
ones, but in a smaller size and with slightly different proportions.
The 'font-variant' property selects that font.
A value of 'normal' selects a font that is not a small-caps font,
'small-caps' selects a small-caps font. It is acceptable (but not
required) in CSS 2.1 if the small-caps font is a created by taking a
normal font and replacing the lower case letters by scaled uppercase
characters. As a last resort, uppercase letters will be used as
replacement for a small-caps font.
The following example results in an 'H3' element in small-caps, with
any emphasized words in oblique, and any emphasized words within an
'H3' oblique small-caps:
h3 { font-variant: small-caps }
em { font-style: oblique }
There may be other variants in the font family as well, such as fonts
with old-style numerals, small-caps numerals, condensed or expanded
letters, etc. CSS 2.1 has no properties that select those.
Note: insofar as this property causes text to be transformed to
uppercase, the same considerations as for 'text-transform' apply.
15.6 Font boldness: the 'font-weight' property
'font-weight'
Value: normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500
| 600 | 700 | 800 | 900 | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see text
The 'font-weight' property selects the weight of the font. The values
'100' to '900' form an ordered sequence, where each number indicates a
weight that is at least as dark as its predecessor. The keyword
'normal' is synonymous with '400', and 'bold' is synonymous with '700'.
Keywords other than 'normal' and 'bold' have been shown to be often
confused with font names and a numerical scale was therefore chosen for
the 9-value list.
p { font-weight: normal } /* 400 */
h1 { font-weight: 700 } /* bold */
The 'bolder' and 'lighter' values select font weights that are relative
to the weight inherited from the parent:
strong { font-weight: bolder }
Fonts (the font data) typically have one or more properties whose
values are names that are descriptive of the "weight" of a font. There
is no accepted, universal meaning to these weight names. Their primary
role is to distinguish faces of differing darkness within a single font
family. Usage across font families is quite variant; for example, a
font that one might think of as being bold might be described as being
Regular, Roman, Book, Medium, Semi- or DemiBold, Bold, or Black,
depending on how black the "normal" face of the font is within the
design. Because there is no standard usage of names, the weight
property values in CSS 2.1 are given on a numerical scale in which the
value '400' (or 'normal') corresponds to the "normal" text face for
that family. The weight name associated with that face will typically
be Book, Regular, Roman, Normal or sometimes Medium.
The association of other weights within a family to the numerical
weight values is intended only to preserve the ordering of darkness
within that family. However, the following heuristics tell how the
assignment is done in typical cases:
* If the font family already uses a numerical scale with nine values
(like e.g. OpenType does), the font weights should be mapped
directly.
* If there is both a face labeled Medium and one labeled Book,
Regular, Roman or Normal, then the Medium is normally assigned to
the '500'.
* The font labeled "Bold" will often correspond to the weight value
'700'.
* If there are fewer then 9 weights in the family, the default
algorithm for filling the "holes" is as follows. If '500' is
unassigned, it will be assigned the same font as '400'. If any of
the values '600', '700', '800' or '900' remains unassigned, they
are assigned to the same face as the next darker assigned keyword,
if any, or the next lighter one otherwise. If any of '300', '200'
or '100' remains unassigned, it is assigned to the next lighter
assigned keyword, if any, or the next darker otherwise.
The following two examples show typical mappings.
Assume four weights in the "Rattlesnake" family, from lightest to
darkest: Regular, Medium, Bold, Heavy.
CAPTION: First example of font-weight mapping
Available faces Assignments Filling the holes
"Rattlesnake Regular" 400 100, 200, 300
"Rattlesnake Medium" 500
"Rattlesnake Bold" 700 600
"Rattlesnake Heavy" 800 900
Assume six weights in the "Ice Prawn" family: Book, Medium, Bold,
Heavy, Black, ExtraBlack. Note that in this instance the user agent has
decided not to assign a numeric value to "Ice Prawn ExtraBlack".
CAPTION: Second example of font-weight mapping
Available faces Assignments Filling the holes
"Ice Prawn Book" 400 100, 200, 300
"Ice Prawn Medium" 500
"Ice Prawn Bold" 700 600
"Ice Prawn Heavy" 800
"Ice Prawn Black" 900
"Ice Prawn ExtraBlack" (none)
Since the intent of the relative keywords 'bolder' and 'lighter' is to
darken or lighten the face within the family and because a family may
not have faces aligned with all the symbolic weight values, the
matching of 'bolder' is to the next darker face available on the client
within the family and the matching of 'lighter' is to the next lighter
face within the family. To be precise, the meaning of the relative
keywords 'bolder' and 'lighter' is as follows:
* 'bolder' selects the next weight that is assigned to a font that is
darker than the inherited one.
* 'lighter' is similar, but works in the opposite direction: it
selects the next lighter keyword with a different font from the
inherited one.
There is no guarantee that there will be a darker face for each of the
'font-weight' values; for example, some fonts may have only a normal
and a bold face, while others may have eight face weights. There is no
guarantee on how a UA will map font faces within a family to weight
values. The only guarantee is that a face of a given value will be no
less dark than the faces of lighter values.
Note: A set of nested elements that mix 'bolder' and 'lighter' will
give unpredictable results depending on the UA, OS, and font
availability. This behavior will be more precisely defined in CSS3.
CSS 2.1 does not specify how the computed value of font-weight is
represented internally or externally.
15.7 Font size: the 'font-size' property
'font-size'
Value: | | | |
inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Media: visual
Computed value: absolute length
The font size corresponds to the em square, a concept used in
typography. Note that certain glyphs may bleed outside their em
squares. Values have the following meanings:
An keyword is an index to a table of font sizes
computed and kept by the UA. Possible values are:
[ xx-small | x-small | small | medium | large | x-large |
xx-large ]
The following table provides user agent guidelines for the
absolute-size mapping to HTML heading and absolute font-sizes.
The 'medium' value is the user's preferred font size and is used
as the reference middle value.
CSS absolute-size values xx-small x-small small medium large x-large
xx-large
HTML font sizes 1 2 3 4 5 6 7
Note: implementation experience has demonstrated that a fixed
ratio between adjacent absolute-size keywords is problematic,
and this specification does NOT recommend such a fixed ratio, in
contrast to previous specifications (CSS1 suggested 1.5, and
CSS2 suggested 1.2).
Implementors should build a table of scaling factors for
absolute-size keywords relative to the 'medium' font size and
the particular device and its characteristics (e.g. the
resolution of the device).
Different media may need different scaling factors. Also, the UA
should take the quality and availability of fonts into account
when computing the table. The table may be different from one
font family to another.
Note 1. To preserve readability, a UA applying these guidelines
should nevertheless avoid creating font-size resulting in less
than 9 pixels per EM unit on a computer display.
Note 2. In CSS1, the suggested scaling factor between adjacent
indexes was 1.5 which user experience proved to be too large. In
CSS2, the suggested scaling factor for computer screen between
adjacent indexes was 1.2 which still created issues for the
small sizes. The new scaling factor varies between each index to
provide better readability.
A keyword is interpreted relative to the table
of font sizes and the font size of the parent element. Possible
values are: [ larger | smaller ]. For example, if the parent
element has a font size of 'medium', a value of 'larger' will
make the font size of the current element be 'large'. If the
parent element's size is not close to a table entry, the UA is
free to interpolate between table entries or round off to the
closest one. The UA may have to extrapolate table values if the
numerical value goes beyond the keywords.
Length and percentage values should not take the font size table into
account when calculating the font size of the element.
Negative values are not allowed.
On all other properties, 'em' and 'ex' length values refer to the
computed font size of the current element. On the 'font-size' property,
these length units refer to the computed font size of the parent
element.
Note that an application may reinterpret an explicit size, depending on
the context. E.g., inside a VR scene a font may get a different size
because of perspective distortion.
Examples:
p { font-size: 16px; }
@media print {
p { font-size: 12pt; }
}
blockquote { font-size: larger }
em { font-size: 150% }
em { font-size: 1.5em }
15.8 Shorthand font property: the 'font' property
'font'
Value: [ [ <'font-style'> || <'font-variant'> || <'font-weight'> ]?
<'font-size'> [ / <'line-height'> ]? <'font-family'> ] | caption | icon
| menu | message-box | small-caption | status-bar | inherit
Initial: see individual properties
Applies to: all elements
Inherited: yes
Percentages: see individual properties
Media: visual
Computed value: see individual properties
The 'font' property is, except as described below, a shorthand property
for setting 'font-style', 'font-variant', 'font-weight', 'font-size',
'line-height' and 'font-family' at the same place in the style sheet.
The syntax of this property is based on a traditional typographical
shorthand notation to set multiple properties related to fonts.
All font-related properties are first reset to their initial values,
including those listed in the preceding paragraph. Then, those
properties that are given explicit values in the 'font' shorthand are
set to those values. For a definition of allowed and initial values,
see the previously defined properties.
p { font: 12px/14px sans-serif }
p { font: 80% sans-serif }
p { font: x-large/110% "New Century Schoolbook", serif }
p { font: bold italic large Palatino, serif }
p { font: normal small-caps 120%/120% fantasy }
In the second rule, the font size percentage value ('80%') refers to
the font size of the parent element. In the third rule, the line height
percentage refers to the font size of the element itself.
In the first three rules above, the 'font-style', 'font-variant' and
'font-weight' are not explicitly mentioned, which means they are all
three set to their initial value ('normal'). The fourth rule sets the
'font-weight' to 'bold', the 'font-style' to 'italic' and implicitly
sets 'font-variant' to 'normal'.
The fifth rule sets the 'font-variant' ('small-caps'), the 'font-size'
(120% of the parent's font), the 'line-height' (120% times the font
size) and the 'font-family' ('fantasy'). It follows that the keyword
'normal' applies to the two remaining properties: 'font-style' and
'font-weight'.
The following values refer to system fonts:
caption
The font used for captioned controls (e.g., buttons, drop-downs,
etc.).
icon
The font used to label icons.
menu
The font used in menus (e.g., dropdown menus and menu lists).
message-box
The font used in dialog boxes.
small-caption
The font used for labeling small controls.
status-bar
The font used in window status bars.
System fonts may only be set as a whole; that is, the font family,
size, weight, style, etc. are all set at the same time. These values
may then be altered individually if desired. If no font with the
indicated characteristics exists on a given platform, the user agent
should either intelligently substitute (e.g., a smaller version of the
'caption' font might be used for the 'small-caption' font), or
substitute a user agent default font. As for regular fonts, if, for a
system font, any of the individual properties are not part of the
operating system's available user preferences, those properties should
be set to their initial values.
That is why this property is "almost" a shorthand property: system
fonts can only be specified with this property, not with 'font-family'
itself, so 'font' allows authors to do more than the sum of its
subproperties. However, the individual properties such as 'font-weight'
are still given values taken from the system font, which can be
independently varied.
Example(s):
button { font: 300 italic 1.3em/1.7em "FB Armada", sans-serif }
button p { font: menu }
button p em { font-weight: bolder }
If the font used for dropdown menus on a particular system happened to
be, for example, 9-point Charcoal, with a weight of 600, then P
elements that were descendants of BUTTON would be displayed as if this
rule were in effect:
button p { font: 600 9px Charcoal }
Because the 'font' shorthand property resets any property not
explicitly given a value to its initial value, this has the same effect
as this declaration:
button p {
font-family: Charcoal;
font-style: normal;
font-variant: normal;
font-weight: 600;
font-size: 9px;
line-height: normal;
}
__________________________________________________________________
__________________________________________________________________
16 Text
Contents
* 16.1 Indentation: the 'text-indent' property
* 16.2 Alignment: the 'text-align' property
* 16.3 Decoration
+ 16.3.1 Underlining, overlining, striking, and blinking: the
'text-decoration' property
* 16.4 Letter and word spacing: the 'letter-spacing' and
'word-spacing' properties
* 16.5 Capitalization: the 'text-transform' property
* 16.6 White space: the 'white-space' property
+ 16.6.1 The 'white-space' processing model
+ 16.6.2 Example of bidirectionality with white space collapsing
+ 16.6.3 Control and combining characters' details
The properties defined in the following sections affect the visual
presentation of characters, spaces, words, and paragraphs.
16.1 Indentation: the 'text-indent' property
'text-indent'
Value: | | inherit
Initial: 0
Applies to: block-level elements, table cells and inline blocks
Inherited: yes
Percentages: refer to width of containing block
Media: visual
Computed value: the percentage as specified or the absolute length
This property specifies the indentation of the first line of text in a
block. More precisely, it specifies the indentation of the first box
that flows into the block's first line box. The box is indented with
respect to the left (or right, for right-to-left layout) edge of the
line box. User agents should render this indentation as blank space.
Values have the following meanings:
The indentation is a fixed length.
The indentation is a percentage of the containing block width.
The value of 'text-indent' may be negative, but there may be
implementation-specific limits. If the value of 'text-indent' is either
negative or exceeds the width of the block, that first box, described
above, can overflow the block. The value of 'overflow' will affect
whether such text that overflows the block is visible.
Example(s):
The following example causes a '3em' text indent.
p { text-indent: 3em }
Note: Since the 'text-indent' property inherits, when specified on a
block element, it will affect descendent inline-block elements. For
this reason, it is often wise to specify 'text-indent: 0' on elements
that are specified 'display:inline-block'.
16.2 Alignment: the 'text-align' property
'text-align'
Value: left | right | center | justify | inherit
Initial: a nameless value that acts as 'left' if 'direction' is
'ltr', 'right' if 'direction' is 'rtl'
Applies to: block-level elements, table cells and inline blocks
Inherited: yes
Percentages: N/A
Media: visual
Computed value: the initial value or as specified
This property describes how inline content of a block is aligned.
Values have the following meanings:
left, right, center, justify
Left, right, center, and justify text, respectively, as
described in the section on inline formatting.
A block of text is a stack of line boxes. In the case of 'left',
'right' and 'center', this property specifies how the inline boxes
within each line box align with respect to the line box's left and
right sides; alignment is not with respect to the viewport. In the case
of 'justify', this property specifies that the inline boxes are to be
made flush with both sides of the block. (See also 'letter-spacing' and
'word-spacing'.)
If the computed value of text-align is 'justify' while the computed
value of white-space is 'pre' or 'pre-line', the actual value of
text-align is set to the initial value.
Example(s):
In this example, note that since 'text-align' is inherited, all
block-level elements inside DIV elements with a class name of
'important' will have their inline content centered.
div.important { text-align: center }
Note. The actual justification algorithm used depends on the user-agent
and the language/script of the text.
Conforming user agents may interpret the value 'justify' as 'left' or
'right', depending on whether the element's default writing direction
is left-to-right or right-to-left, respectively.
16.3 Decoration
16.3.1 Underlining, overlining, striking, and blinking: the 'text-decoration'
property
'text-decoration'
Value: none | [ underline || overline || line-through || blink ] |
inherit
Initial: none
Applies to: all elements
Inherited: no (see prose)
Percentages: N/A
Media: visual
Computed value: as specified
This property describes decorations that are added to the text of an
element using the element's color. When specified on an inline element,
it affects all the boxes generated by that element; for all other
elements, the decorations are propagated to an anonymous inline box
that wraps all the in-flow inline children of the element, and to any
block-level in-flow descendants. It is not, however, further propagated
to floating and absolutely positioned descendants, nor to the contents
of 'inline-table' and 'inline-block' descendants.
Underlines, overlines, and line-throughs are applied only to text
(including white space, letter spacing, and word spacing): margins,
borders, and padding are skipped. If an element contains no text, user
agents must refrain from rendering these text decorations on the
element. For example, images will not be underlined.
The 'text-decoration' property on descendant elements cannot have any
effect on the decoration of the ancestor. In determining the position
of and thickness of text decoration lines, user agents may consider the
font sizes of and dominant baselines of descendants, but must use the
same baseline and thickness on each line. Relatively positioning a
descendant moves all text decorations affecting it along with the
descendant's text; it does not affect calculation of the decoration's
initial position on that line.
Values have the following meanings:
none
Produces no text decoration.
underline
Each line of text is underlined.
overline
Each line of text has a line above it.
line-through
Each line of text has a line through the middle.
blink
Text blinks (alternates between visible and invisible).
Conforming user agents may simply not blink the text. Note that
not blinking the text is one technique to satisfy checkpoint 3.3
of WAI-UAAG.
The color(s) required for the text decoration must be derived from the
'color' property value of the element on which 'text-decoration' is
set. The color of decorations must remain the same even if descendant
elements have different 'color' values.
Some user agents have implemented text-decoration by propagating the
decoration to the descendant elements as opposed to preserving a
constant thickness and line position as described above. This was
arguably allowed by the looser wording in CSS2. SVG1, CSS1-only, and
CSS2-only user agents may implement the older model and still claim
conformance to this part of CSS 2.1. (This does not apply to UAs
developed after this specification was released.)
Example(s):
In the following example for HTML, the text content of all A elements
acting as hyperlinks (whether visited or not) will be underlined:
a:visited,a:link { text-decoration: underline }
Example(s):
In the following style sheet and document fragment:
blockquote { text-decoration: underline; color: blue; }
em { display: block; }
cite { color: fuchsia; }
Help, help!
I am under a hat!
--GwieF
...the underlining for the blockquote element is propagated to an
anonymous inline element that surrounds the span element, causing the
text "Help, help!" to be blue, with the blue underlining from the
anonymous inline underneath it, the color being taken from the
blockquote element. The text in the em block is also
underlined, as it is in an in-flow block to which the underline is
propagated. The final line of text is fuchsia, but the underline
underneath it is still the blue underline from the anonymous inline
element.
Sample rendering of the above underline example
This diagram shows the boxes involved in the example above. The rounded
aqua line represents the anonymous inline element wrapping the inline
contents of the paragraph element, the rounded blue line represents the
span element, and the orange lines represent the blocks.
16.4 Letter and word spacing: the 'letter-spacing' and 'word-spacing' properties
'letter-spacing'
Value: normal | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: 'normal' or absolute length
This property specifies spacing behavior between text characters.
Values have the following meanings:
normal
The spacing is the normal spacing for the current font. This
value allows the user agent to alter the space between
characters in order to justify text.
This value indicates inter-character space in addition to the
default space between characters. Values may be negative, but
there may be implementation-specific limits. User agents may not
further increase or decrease the inter-character space in order
to justify text.
Character spacing algorithms are user agent-dependent.
Example(s):
In this example, the space between characters in BLOCKQUOTE elements is
increased by '0.1em'.
blockquote { letter-spacing: 0.1em }
In the following example, the user agent is not permitted to alter
inter-character space:
blockquote { letter-spacing: 0cm } /* Same as '0' */
When the resultant space between two characters is not the same as the
default space, user agents should not use ligatures.
'word-spacing'
Value: normal | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: for 'normal' the value '0'; otherwise the absolute
length
This property specifies spacing behavior between words. Values have the
following meanings:
normal
The normal inter-word space, as defined by the current font
and/or the UA.
This value indicates inter-word space in addition to the default
space between words. Values may be negative, but there may be
implementation-specific limits.
Word spacing algorithms are user agent-dependent. Word spacing is also
influenced by justification (see the 'text-align' property). Word
spacing affects each space (U+0020), non-breaking space (U+00A0), and
ideographic space (U+3000) left in the text after the white space
processing rules have been applied.
Example(s):
In this example, the word-spacing between each word in H1 elements is
increased by '1em'.
h1 { word-spacing: 1em }
16.5 Capitalization: the 'text-transform' property
'text-transform'
Value: capitalize | uppercase | lowercase | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property controls capitalization effects of an element's text.
Values have the following meanings:
capitalize
Puts the first character of each word in uppercase; other
characters are unaffected.
uppercase
Puts all characters of each word in uppercase.
lowercase
Puts all characters of each word in lowercase.
none
No capitalization effects.
The actual transformation in each case is written language dependent.
See RFC 3066 ([RFC3066]) for ways to find the language of an element.
Only characters belonging to "bicameral scripts" [UNICODE] are
affected.
Example(s):
In this example, all text in an H1 element is transformed to uppercase
text.
h1 { text-transform: uppercase }
16.6 White space: the 'white-space' property
'white-space'
Value: normal | pre | nowrap | pre-wrap | pre-line | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property declares how white space inside the element is handled.
Values have the following meanings:
normal
This value directs user agents to collapse sequences of white
space, and break lines as necessary to fill line boxes.
pre
This value prevents user agents from collapsing sequences of
white space. Lines are only broken at newlines in the source, or
at occurrences of "\A" in generated content.
nowrap
This value collapses white space as for 'normal', but suppresses
line breaks within text.
pre-wrap
This value prevents user agents from collapsing sequences of
white space. Lines are broken at newlines in the source, at
occurrences of "\A" in generated content, and as necessary to
fill line boxes.
pre-line
This value directs user agents to collapse sequences of white
space. Lines are broken at newlines in the source, at
occurrences of "\A" in generated content, and as necessary to
fill line boxes.
Newlines in the source can be represented by a carriage return
(U+000D), a linefeed (U+000A) or both (U+000D U+000A) or by some other
mechanism that identifies the beginning and end of document segments,
such as the SGML RECORD-START and RECORD-END tokens. The CSS
'white-space' processing model assumes all newlines have been
normalized to line feeds.
Example(s):
The following examples show what white space behavior is expected from
the PRE and P elements and the "nowrap" attribute in HTML.
pre { white-space: pre }
p { white-space: normal }
td[nowrap] { white-space: nowrap }
In addition, the effect of an HTML PRE element with the non-standard
"wrap" attribute is demonstrated by the following example:
pre[wrap] { white-space: pre-wrap }
16.6.1 The 'white-space' processing model
Any text that is directly contained inside a block element (not inside
an inline element) should be treated as an anonymous inline element.
For each inline element (including anonymous inlines elements), the
following steps are performed, treating bidi formatting characters as
if they were not there:
1. Each tab (U+0009), carriage return (U+000D), or space (U+0020)
character surrounding a linefeed (U+000A) character is removed if
'white-space' is set to 'normal', 'nowrap', or 'pre-line'.
2. If 'white-space' is set to 'pre' or 'pre-wrap', any sequence of
spaces (U+0020) unbroken by an element boundary is treated as a
sequence of non-breaking spaces. However, for 'pre-wrap', a line
breaking opportunity exists at the end of the sequence.
3. If 'white-space' is set to 'normal' or 'nowrap', linefeed
characters are transformed for rendering purpose into one of the
following characters: a space character, a zero width space
character (U+200B), or no character (i.e. not rendered), according
to UA-specific algorithms based on the content script.
4. If 'white-space' is set to 'normal', 'nowrap', or 'pre-line',
1. every tab (U+0009) is converted to a space (U+0020)
2. any space (U+0020) following another space (U+0020) -- even a
space before the inline, if that space also has 'white-space'
set to 'normal', 'nowrap' or 'pre-line' -- is removed.
Then, the entire block is rendered. Inlines are laid out, taking bidi
reordering into account, and wrapping as specified by the 'white-space'
property. When wrapping, line breaking opportunities are determined
based on the text prior to the white space collapsing steps above.
As each line is laid out,
1. If a space (U+0020) at the beginning of a line has 'white-space'
set to 'normal', 'nowrap', or 'pre-line', it is removed.
2. All tabs (U+0009) are rendered as a horizontal shift that lines up
the start edge of the next glyph with the next tab stop. Tab stops
occur at points that are mutiples of 8 times the width of a space
(U+0020) rendered in the block's font from the block's starting
content edge.
3. If a space (U+0020) at the end of a line has 'white-space' set to
'normal', 'nowrap', or 'pre-line', it is also removed.
4. If spaces (U+0020) or tabs (U+0009) at the end of a line have
'white-space' set to 'pre-wrap', UAs may visually collapse them.
A float should not introduce a line break opportunity.
Note. CSS 2.1 does not fully define where line breaking opportunities
occur. Floated and absolutely-positioned elements do not introduce a
line breaking opportunity.
16.6.2 Example of bidirectionality with white space collapsing
Given the following markup fragment, taking special note of spaces
(with varied backgrounds and borders for emphasis and identification):
A B C
...where the element represents a left-to-right embedding and the
element represents a right-to-left embedding, and assuming that
the 'white-space' property is set to 'normal', the above processing
model would result in the following:
* The space before the B ( ) would collapse with the space after the
A ( ).
* The space before the C ( ) would collapse with the space after the
B ( ).
This would leave two spaces, one after the A in the left-to-right
embedding level, and one after the B in the right-to-left embedding
level. This is then rendered according to the Unicode bidirectional
algorithm, with the end result being:
A BC
Note that there are two spaces between A and B, and none between B and
C. This can sometimes be avoided by using the natural bidirectionality
of characters instead of explicit embedding levels. Also, it is good to
avoid spaces immediately inside start and end tags, as these tend to do
weird things when dealing with white space collapsing.
16.6.3 Control and combining characters' details
Control characters other than U+0009 (tab), U+000A (line feed), U+0020
(space), and U+202x (bidi formatting characters) are treated as
characters to render in the same way as any normal character.
Combining characters should be treated as part of the character with
which they are supposed to combine. For example, :first-letter styles
the entire glyph if you have content like "ö"; it
doesn't just match the base character.
__________________________________________________________________
__________________________________________________________________
17 Tables
Contents
* 17.1 Introduction to tables
* 17.2 The CSS table model
+ 17.2.1 Anonymous table objects
* 17.3 Columns
* 17.4 Tables in the visual formatting model
+ 17.4.1 Caption position and alignment
* 17.5 Visual layout of table contents
+ 17.5.1 Table layers and transparency
+ 17.5.2 Table width algorithms: the 'table-layout' property
o 17.5.2.1 Fixed table layout
o 17.5.2.2 Automatic table layout
+ 17.5.3 Table height algorithms
+ 17.5.4 Horizontal alignment in a column
+ 17.5.5 Dynamic row and column effects
* 17.6 Borders
+ 17.6.1 The separated borders model
o 17.6.1.1 Borders and Backgrounds around empty cells: the
'empty-cells' property
+ 17.6.2 The collapsing border model
o 17.6.2.1 Border conflict resolution
+ 17.6.3 Border styles
17.1 Introduction to tables
This chapter defines the processing model for tables in CSS. Part of
this processing model is the layout. For the layout, this chapter
introduces two algorithms; the first, the fixed table layout algorithm,
is well-defined, but the second, the automatic table layout algorithm,
is not fully defined by this specification.
For the automatic table layout algorithm, some widely deployed
implementations have achieved relatively close interoperability.
Table layout can be used to represent tabular relationships between
data. Authors specify these relationships in the document language and
can specify their presentation using CSS 2.1.
In a visual medium, CSS tables can also be used to achieve specific
layouts. In this case, authors should not use table-related elements in
the document language, but should apply the CSS to the relevant
structural elements to achieve the desired layout.
Authors may specify the visual formatting of a table as a rectangular
grid of cells. Rows and columns of cells may be organized into row
groups and column groups. Rows, columns, row groups, column groups, and
cells may have borders drawn around them (there are two border models
in CSS 2.1). Authors may align data vertically or horizontally within a
cell and align data in all cells of a row or column.
Example(s):
Here is a simple three-row, three-column table described in HTML 4:
This is a simple 3x3 table
Header 1
Cell 1
Cell 2
Header 2
Cell 3
Cell 4
Header 3
Cell 5
Cell 6
This code creates one table (the TABLE element), three rows (the TR
elements), three header cells (the TH elements), and six data cells
(the TD elements). Note that the three columns of this example are
specified implicitly: there are as many columns in the table as
required by header and data cells.
The following CSS rule centers the text horizontally in the header
cells and presents the text in the header cells with a bold font
weight:
th { text-align: center; font-weight: bold }
The next rules align the text of the header cells on their baseline and
vertically center the text in each data cell:
th { vertical-align: baseline }
td { vertical-align: middle }
The next rules specify that the top row will be surrounded by a 3px
solid blue border and each of the other rows will be surrounded by a
1px solid black border:
table { border-collapse: collapse }
tr#row1 { border: 3px solid blue }
tr#row2 { border: 1px solid black }
tr#row3 { border: 1px solid black }
Note, however, that the borders around the rows overlap where the rows
meet. What color (black or blue) and thickness (1px or 3px) will the
border between row1 and row2 be? We discuss this in the section on
border conflict resolution.
The following rule puts the table caption above the table:
caption { caption-side: top }
The preceding example shows how CSS works with HTML 4 elements; in HTML
4, the semantics of the various table elements (TABLE, CAPTION, THEAD,
TBODY, TFOOT, COL, COLGROUP, TH, and TD) are well-defined. In other
document languages (such as XML applications), there may not be
pre-defined table elements. Therefore, CSS 2.1 allows authors to "map"
document language elements to table elements via the 'display'
property. For example, the following rule makes the FOO element act
like an HTML TABLE element and the BAR element act like a CAPTION
element:
FOO { display : table }
BAR { display : table-caption }
We discuss the various table elements in the following section. In this
specification, the term table element refers to any element involved in
the creation of a table. An "internal" table element is one that
produces a row, row group, column, column group, or cell.
17.2 The CSS table model
The CSS table model is based on the HTML4 table model, in which the
structure of a table closely parallels the visual layout of the table.
In this model, a table consists of an optional caption and any number
of rows of cells. The table model is said to be "row primary" since
authors specify rows, not columns, explicitly in the document language.
Columns are derived once all the rows have been specified -- the first
cell of each row belongs to the first column, the second to the second
column, etc.). Rows and columns may be grouped structurally and this
grouping reflected in presentation (e.g., a border may be drawn around
a group of rows).
Thus, the table model consists of tables, captions, rows, row groups,
columns, column groups, and cells.
The CSS model does not require that the document language include
elements that correspond to each of these components. For document
languages (such as XML applications) that do not have pre-defined table
elements, authors must map document language elements to table
elements; this is done with the 'display' property. The following
'display' values assign table formatting rules to an arbitrary element:
table (In HTML: TABLE)
Specifies that an element defines a block-level table: it is a
rectangular block that participates in a block formatting
context.
inline-table (In HTML: TABLE)
Specifies that an element defines an inline-level table: it is a
rectangular block that participates in an inline formatting
context).
table-row (In HTML: TR)
Specifies that an element is a row of cells.
table-row-group (In HTML: TBODY)
Specifies that an element groups one or more rows.
table-header-group (In HTML: THEAD)
Like 'table-row-group', but for visual formatting, the row group
is always displayed before all other rows and row groups and
after any top captions. Print user agents may repeat header rows
on each page spanned by a table. If a table contains multiple
elements with 'display: table-header-group', only the first is
rendered as a header; the others are treated as if they had
'display: table-row-group'.
table-footer-group (In HTML: TFOOT)
Like 'table-row-group', but for visual formatting, the row group
is always displayed after all other rows and row groups and
before any bottom captions. Print user agents may repeat footer
rows on each page spanned by a table. If a table contains
multiple elements with 'display: table-footer-group', only the
first is rendered as a footer; the others are treated as if they
had 'display: table-row-group'.
table-column (In HTML: COL)
Specifies that an element describes a column of cells.
table-column-group (In HTML: COLGROUP)
Specifies that an element groups one or more columns.
table-cell (In HTML: TD, TH)
Specifies that an element represents a table cell.
table-caption (In HTML: CAPTION)
Specifies a caption for the table. All elements with 'display:
table-caption' must be rendered, as described in section 17.4.
Replaced elements with these 'display' values are treated as their
given display types during layout. For example, an image that is set to
'display: table-cell' will fill the available cell space, and its
dimensions might contribute towards the table sizing algorithms, as
with an ordinary cell.
Elements with 'display' set to 'table-column' or 'table-column-group'
are not rendered (exactly as if they had 'display: none'), but they are
useful, because they may have attributes which induce a certain style
for the columns they represent.
The default style sheet for HTML4 in the appendix illustrates the use
of these values for HTML4:
table { display: table }
tr { display: table-row }
thead { display: table-header-group }
tbody { display: table-row-group }
tfoot { display: table-footer-group }
col { display: table-column }
colgroup { display: table-column-group }
td, th { display: table-cell }
caption { display: table-caption }
User agents may ignore these 'display' property values for HTML table
elements, since HTML tables may be rendered using other algorithms
intended for backwards compatible rendering. However, this is not meant
to discourage the use of 'display: table' on other, non-table elements
in HTML.
17.2.1 Anonymous table objects
Document languages other than HTML may not contain all the elements in
the CSS 2.1 table model. In these cases, the "missing" elements must be
assumed in order for the table model to work. Any table element will
automatically generate necessary anonymous table objects around itself,
consisting of at least three nested objects corresponding to a
'table'/'inline-table' element, a 'table-row' element, and a
'table-cell' element. Missing elements generate anonymous objects
(e.g., anonymous boxes in visual table layout) according to the
following rules:
1. If the parent P of a 'table-cell' box T is not a 'table-row', a box
corresponding to a 'table-row' will be generated between P and T.
This box will span all consecutive 'table-cell' sibling boxes of T.
2. If the parent P of a 'table-row' box T is not a 'table',
'inline-table', 'table-header-group', 'table-footer-group' or
'table-row-group' box, a box corresponding to a 'table' element
will be generated between P and T. If P is an 'inline' box, then
the generated box must be an 'inline-table' box instead of a
'table' box. This box will span all consecutive sibling boxes of T
that require a 'table' parent: 'table-row', 'table-row-group',
'table-header-group', 'table-footer-group', 'table-column',
'table-column-group', and 'table-caption'. T and T's siblings may
also be anonymous 'table-row' boxes generated by rule 1.
3. If the parent P of a 'table-column' box T is not a 'table',
'inline-table', or 'table-column-group' box, a box corresponding to
a 'table' element will be generated between P and T. If P is an
'inline' box, then the generated box must be an 'inline-table' box
instead of a 'table' box. This box will span all consecutive
sibling boxes of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption',
including any anonymous 'table-row' boxes generated by rule 1.
4. If the parent P of a 'table-row-group' (or 'table-header-group',
'table-footer-group', or 'table-column-group' or 'table-caption')
box T is not a 'table' or 'inline-table', a box corresponding to a
'table' element will be generated between P and T. If P is an
'inline' box, then the generated box must be an 'inline-table' box
instead of a 'table' box. This box will span all consecutive
sibling boxes of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption',
including any anonymous 'table-row' boxes generated by rule 1.
5. If a child T of a 'table', 'inline-table', 'table-row-group',
'table-header-group', 'table-footer-group', or 'table-row' box is
an anonymous inline box that contains only white space, then it is
treated as if it had 'display: none'.
6. If a child T of a 'table' box (or 'inline-table') P is not a
'table-row-group', 'table-header-group', 'table-footer-group',
'table-caption', 'table-column', 'table-column-group' or
'table-row' box, a box corresponding to a 'table-row' element will
be generated between P and T. This box spans all consecutive
siblings of T that are not 'table-row-group', 'table-header-group',
'table-footer-group', 'table-caption', 'table-column',
'table-column-group' or 'table-row' boxes.
7. If a child T of a 'table-row-group' box (or 'table-header-group' or
'table-footer-group') P is not a 'table-row' box, a box
corresponding to a 'table-row' element will be generated between P
and T. This box spans all consecutive siblings of T that are not
'table-row' boxes.
8. If a child T of a 'table-row' box P is not a 'table-cell' box, a
box corresponding to a 'table-cell' element will be generated
between P and T. This box spans all consecutive siblings of T that
are not 'table-cell' boxes.
Example(s):
In this XML example, a 'table' element is assumed to contain the HBOX
element:
George42871998
because the associated style sheet is:
HBOX { display: table-row }
VBOX { display: table-cell }
Example(s):
In this example, three 'table-cell' elements are assumed to contain the
text in the ROWs. Note that the text is further encapsulated in
anonymous inline boxes, as explained in visual formatting model:
This is the top row.This is the middle row.This is the bottom row.
The style sheet is:
STACK { display: inline-table }
ROW { display: table-row }
D { display: inline; font-weight: bolder }
17.3 Columns
Table cells may belong to two contexts: rows and columns. However, in
the source document cells are descendants of rows, never of columns.
Nevertheless, some aspects of cells can be influenced by setting
properties on columns.
The following properties apply to column and column-group elements:
'border'
The various border properties apply to columns only if
'border-collapse' is set to 'collapse' on the table element. In
that case, borders set on columns and column groups are input to
the conflict resolution algorithm that selects the border styles
at every cell edge.
'background'
The background properties set the background for cells in the
column, but only if both the cell and row have transparent
backgrounds. See "Table layers and transparency."
'width'
The 'width' property gives the minimum width for the column.
'visibility'
If the 'visibility' of a column is set to 'collapse', none of
the cells in the column are rendered, and cells that span into
other columns are clipped. In addition, the width of the table
is diminished by the width the column would have taken up. See
"Dynamic effects" below. Other values for 'visibility' have no
effect.
Example(s):
Here are some examples of style rules that set properties on columns.
The first two rules together implement the "rules" attribute of HTML 4
with a value of "cols". The third rule makes the "totals" column blue,
the final two rules shows how to make a column a fixed size, by using
the fixed layout algorithm.
col { border-style: none solid }
table { border-style: hidden }
col.totals { background: blue }
table { table-layout: fixed }
col.totals { width: 5em }
17.4 Tables in the visual formatting model
In terms of the visual formatting model, a table can behave like a
block-level (for 'display: table') or inline-level (for 'display:
inline-table') element.
In both cases, the table box generates an anonymous box that contains
the table box itself and any caption boxes (in document order). The
caption boxes are block-level boxes that retain their own content,
padding, margin, and border areas, and are rendered as normal blocks
inside the anonymous box. Whether the caption boxes are placed before
or after the table box is decided by the 'caption-side' property, as
described below.
The anonymous box is a 'block' box if the table is block-level, and an
'inline-block' box if the table is inline-level. The anonymous box
establishes a block formatting context. The table box (not the
anonymous box) is used when doing baseline vertical alignment for an
'inline-table'. The width of the anonymous box is the border-edge width
of the table box inside it, as described by section 17.5.2. Percentages
on 'width' and 'height' on the table are relative to the anonymous
box's containing block, not the anonymous box itself.
The computed values of properties 'position', 'float', 'margin-*',
'top', 'right', 'bottom', and 'left' on the table box are used on the
anonymous box instead of the table box. The table box uses the initial
values for those properties.
A table with a caption above it
Diagram of a table with a caption above it; the top margin of the
caption is collapsed with the top margin of the table.
17.4.1 Caption position and alignment
'caption-side'
Value: top | bottom | inherit
Initial: top
Applies to: 'table-caption' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the caption box with respect to
the table box. Values have the following meanings:
top
Positions the caption box above the table box.
bottom
Positions the caption box below the table box.
Note: CSS2 described a different width and horizontal alignment
behavior. That behavior will be introduced in CSS3 using the values
'top-outside' and 'bottom-outside' on this property.
To align caption content horizontally within the caption box, use the
'text-align' property.
Example(s):
In this example, the 'caption-side' property places captions below
tables. The caption will be as wide as the parent of the table, and
caption text will be left-justified.
caption { caption-side: bottom;
width: auto;
text-align: left }
17.5 Visual layout of table contents
Internal table elements generate rectangular boxes with content and
borders. Cells have padding as well. Internal table elements do not
have margins.
The visual layout of these boxes is governed by a rectangular,
irregular grid of rows and columns. Each box occupies a whole number of
grid cells, determined according to the following rules. These rules do
not apply to HTML 4 or earlier HTML versions; HTML imposes its own
limitations on row and column spans.
1. Each row box occupies one row of grid cells. Together, the row
boxes fill the table from top to bottom in the order they occur in
the source document (i.e., the table occupies exactly as many grid
rows as there are row elements).
2. A row group occupies the same grid cells as the rows it contains.
3. A column box occupies one or more columns of grid cells. Column
boxes are placed next to each other in the order they occur. The
first column box may be either on the left or on the right,
depending on the value of the 'direction' property of the table.
4. A column group box occupies the same grid cells as the columns it
contains.
5. Cells may span several rows or columns. (Although CSS 2.1 doesn't
define how the number of spanned rows or columns is determined, a
user agent may have special knowledge about the source document; a
future update of CSS may provide a way to express this knowledge in
CSS syntax.) Each cell is thus a rectangular box, one or more grid
cells wide and high. The top row of this rectangle is in the row
specified by the cell's parent. The rectangle must be as far to the
left as possible, but the part of the cell in the first column it
occupies must not overlap with any other cell box (i.e., a
row-spanning cell starting in a prior row), and the cell must be to
the right of all cells in the same row that are earlier in the
source document. If this position would cause a column-spanning
cell to overlap a row-spanning cell from a prior row, CSS does not
define the results: implementations may either overlap the cells
(as is done in many HTML implementations) or may shift the later
cell to the right to avoid such overlap. (This constraint holds if
the 'direction' property of the table is 'ltr'; if the 'direction'
is 'rtl', interchange "left" and "right" in the previous two
sentences.)
6. A cell box cannot extend beyond the last row box of a table or
row-group; the user agents must shorten it until it fits.
The edges of the rows, columns, row groups and column groups in the
collapsing borders model coincide with the hypothetical grid lines on
which the borders of the cells are centered. (And thus, in this model,
the rows together exactly cover the table, leaving no gaps; ditto for
the columns.) In the separated borders model, the edges coincide with
the border edges of cells. (And thus, in this model, there may be gaps
between the rows, columns, row groups or column groups, corresponding
to the 'border-spacing' property.)
Note. Positioning and floating of table cells can cause them not to be
table cells anymore, according to the rules in section 9.7. When
floating is used, the rules on anonymous table objects may cause an
anonymous cell object to be created as well.
Here is an example illustrating rule 5. The following illegal (X)HTML
snippet defines conflicting cells:
1
2
3
4
5
User agents are free to visually overlap the cells, as in the figure on
the left, or to shift the cell to avoid the visual overlap, as in the
figure on the right.
One table with overlapping cells and one without [D]
Two possible renderings of an erroneous HTML table.
17.5.1 Table layers and transparency
For the purposes of finding the background of each table cell, the
different table elements may be thought of as being on six superimposed
layers. The background set on an element in one of the layers will only
be visible if the layers above it have a transparent background.
schema of table layers [D]
Schema of table layers.
1. The lowest layer is a single plane, representing the table box
itself. Like all boxes, it may be transparent.
2. The next layer contains the column groups. Each column group
extends from the top of the cells in the top row to the bottom of
the cells on the bottom row and from the left edge of its leftmost
column to the right edge of its rightmost column. The background
covers exactly the full area of all cells that originate in the
column group, even if they span outside the column group, but this
difference in area does not affect background image positioning.
3. On top of the column groups are the areas representing the column
boxes. Each column is as tall as the column groups and as wide as a
normal (single-column-spanning) cell in the column. The background
covers exactly the full area of all cells that originate in the
column, even if they span outside the column, but this difference
in area does not affect background image positioning.
4. Next is the layer containing the row groups. Each row group extends
from the top left corner of its topmost cell in the first column to
the bottom right corner of its bottommost cell in the last column.
5. The next to last layer contains the rows. Each row is as wide as
the row groups and as tall as a normal (single-row-spanning) cell
in the row. As with columns, the background covers exactly the full
area of all cells that originate in the row, even if they span
outside the row, but this difference in area does not affect
background image positioning.
6. The topmost layer contains the cells themselves. As the figure
shows, although all rows contain the same number of cells, not
every cell may have specified content. In the separated borders
model ('border-collapse' is 'separate'), if the value of their
'empty-cells' property is 'hide' these "empty" cells are
transparent through the cell, row, row group, column and column
group backgrounds, letting the table background show through.
A "missing cell" is a cell in the row/column grid that is not occupied
by an element or pseudo-element. Missing cells are rendered as if an
anonymous table-cell box occupied their position in the grid.
In the following example, the first row contains four non-empty cells,
but the second row contains only one non-empty cell, and thus the table
background shines through, except where a cell from the first row spans
into this row. The following HTML code and style rules
Table example
1
2
3
4
5
might be formatted as follows:
Table with three empty cells in bottom row [D]
Table with empty cells in the bottom row.
Note that if the table has 'border-collapse: separate', the background
of the area given by the 'border-spacing' property is always the
background of the table element. See the separated borders model.
17.5.2 Table width algorithms: the 'table-layout' property
CSS does not define an "optimal" layout for tables since, in many
cases, what is optimal is a matter of taste. CSS does define
constraints that user agents must respect when laying out a table. User
agents may use any algorithm they wish to do so, and are free to prefer
rendering speed over precision, except when the "fixed layout
algorithm" is selected.
Note that this section overrides the rules that apply to calculating
widths as described in section 10.3. In particular, if the margins of a
table are set to '0' and the width to 'auto', the table will not
automatically size to fill its containing block. However, once the
calculated value of 'width' for the table is found (using the
algorithms given below or, when appropriate, some other UA dependant
algorithm) then the other parts of section 10.3 do apply. Therefore a
table can be centered using left and right 'auto' margins, for
instance.
Future updates of CSS may introduce ways of making tables automatically
fit their containing blocks.
'table-layout'
Value: auto | fixed | inherit
Initial: auto
Applies to: 'table' and 'inline-table' elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The 'table-layout' property controls the algorithm used to lay out the
table cells, rows, and columns. Values have the following meaning:
fixed
Use the fixed table layout algorithm
auto
Use any automatic table layout algorithm
The two algorithms are described below.
17.5.2.1 Fixed table layout
With this (fast) algorithm, the horizontal layout of the table does not
depend on the contents of the cells; it only depends on the table's
width, the width of the columns, and borders or cell spacing.
The table's width may be specified explicitly with the 'width'
property. A value of 'auto' (for both 'display: table' and 'display:
inline-table') means use the automatic table layout algorithm. However,
if the table is a block-level table ('display: table') in normal flow,
a UA may (but does not have to) use the algorithm of 10.3.3 to compute
a width and apply fixed table layout even if the specified width is
'auto'.
Example(s):
If a UA supports fixed table layout when 'width' is 'auto', the
following will create a table that is 4em narrower than its containing
block:
table { table-layout: fixed;
margin-left: 2em;
margin-right: 2em }
In the fixed table layout algorithm, the width of each column is
determined as follows:
1. A column element with a value other than 'auto' for the 'width'
property sets the width for that column.
2. Otherwise, a cell in the first row with a value other than 'auto'
for the 'width' property determines the width for that column. If
the cell spans more than one column, the width is divided over the
columns.
3. Any remaining columns equally divide the remaining horizontal table
space (minus borders or cell spacing).
The width of the table is then the greater of the value of the 'width'
property for the table element and the sum of the column widths (plus
cell spacing or borders). If the table is wider than the columns, the
extra space should be distributed over the columns.
If a subsequent row has more columns than the greater of the number
determined by the table-column elements and the number determined by
the first row, then additional columns must not be rendered. When using
'table-layout: fixed', authors should not omit columns from the first
row.
In this manner, the user agent can begin to lay out the table once the
entire first row has been received. Cells in subsequent rows do not
affect column widths. Any cell that has content that overflows uses the
'overflow' property to determine whether to clip the overflow content.
17.5.2.2 Automatic table layout
In this algorithm (which generally requires no more than two passes),
the table's width is given by the width of its columns (and intervening
borders). This algorithm reflects the behavior of several popular HTML
user agents at the writing of this specification. UAs are not required
to implement this algorithm to determine the table layout in the case
that 'table-layout' is 'auto'; they can use any other algorithm even if
it results in different behavior.
Input to the automatic table layout must only include the width of the
containing block and the content of, and any CSS properties set on, the
table and any of its descendants.
Note. This may be defined in more detail in CSS3.
The remainder of this section is non-normative.
This algorithm may be inefficient since it requires the user agent to
have access to all the content in the table before determining the
final layout and may demand more than one pass.
Column widths are determined as follows:
1. Calculate the minimum content width (MCW) of each cell: the
formatted content may span any number of lines but may not overflow
the cell box. If the specified 'width' (W) of the cell is greater
than MCW, W is the minimum cell width. A value of 'auto' means that
MCW is the minimum cell width.
Also, calculate the "maximum" cell width of each cell: formatting
the content without breaking lines other than where explicit line
breaks occur.
2. For each column, determine a maximum and minimum column width from
the cells that span only that column. The minimum is that required
by the cell with the largest minimum cell width (or the column
'width', whichever is larger). The maximum is that required by the
cell with the largest maximum cell width (or the column 'width',
whichever is larger).
3. For each cell that spans more than one column, increase the minimum
widths of the columns it spans so that together, they are at least
as wide as the cell. Do the same for the maximum widths. If
possible, widen all spanned columns by approximately the same
amount.
4. For each column group element with a 'width' other than 'auto',
increase the minimum widths of the columns it spans, so that
together they are at least as wide as the column group's 'width'.
This gives a maximum and minimum width for each column. Column widths
influence the final table width as follows:
1. If the 'table' or 'inline-table' element's 'width' property has a
computed value (W) other than 'auto', the property's value as used
for layout is the greater of W and the minimum width required by
all the columns plus cell spacing or borders (MIN). If W is greater
than MIN, the extra width should be distributed over the columns.
2. If the 'table' or 'inline-table' element has 'width: auto', the
table width used for layout is the greater of the table's
containing block width and MIN. However, if the maximum width
required by the columns plus cell spacing or borders (MAX) is less
than that of the containing block, use MAX.
A percentage value for a column width is relative to the table width.
If the table has 'width: auto', a percentage represents a constraint on
the column's width, which a UA should try to satisfy. (Obviously, this
is not always possible: if the column's width is '110%', the constraint
cannot be satisfied.)
Note. In this algorithm, rows (and row groups) and columns (and column
groups) both constrain and are constrained by the dimensions of the
cells they contain. Setting the width of a column may indirectly
influence the height of a row, and vice versa.
17.5.3 Table height algorithms
The height of a table is given by the 'height' property for the 'table'
or 'inline-table' element. A value of 'auto' means that the height is
the sum of the row heights plus any cell spacing or borders. Any other
value is treated as a minimum height. CSS 2.1 does not define how extra
space is distributed when the 'height' property causes the table to be
taller than it otherwise would be. Note. Future updates of CSS may
specify this further.
The height of a 'table-row' element's box is calculated once the user
agent has all the cells in the row available: it is the maximum of the
row's specified 'height' and the minimum height (MIN) required by the
cells. A 'height' value of 'auto' for a 'table-row' means the row
height used for layout is MIN. MIN depends on cell box heights and cell
box alignment (much like the calculation of a line box height).
Percentage heights on table cells, table rows, and table row groups
compute to 'auto'.
In CSS 2.1, the height of a cell box is the maximum of the table cell's
'height' property and the minimum height required by the content (MIN).
A value of 'auto' for 'height' implies that the value MIN will be used
for layout. CSS 2.1 does not define what percentage values of 'height'
refer to when specified for table cells.
CSS 2.1 does not specify how cells that span more than one row affect
row height calculations except that the sum of the row heights involved
must be great enough to encompass the cell spanning the rows.
The 'vertical-align' property of each table cell determines its
alignment within the row. Each cell's content has a baseline, a top, a
middle, and a bottom, as does the row itself. In the context of tables,
values for 'vertical-align' have the following meanings:
baseline
The baseline of the cell is put at the same height as the
baseline of the first of the rows it spans (see below for the
definition of baselines of cells and rows).
top
The top of the cell box is aligned with the top of the first row
it spans.
bottom
The bottom of the cell box is aligned with the bottom of the
last row it spans.
middle
The center of the cell is aligned with the center of the rows it
spans.
sub, super, text-top, text-bottom, ,
These values do not apply to cells; the cell is aligned at the
baseline instead.
The baseline of a cell is the baseline of the first in-flow line box in
the cell, or the first in-flow table-row in the cell, whichever comes
first. If there is no such line box or table-row, the baseline is the
bottom of content edge of the cell box. For the purposes of finding a
baseline, in-flow boxes with a scrolling mechanisms (see the 'overflow'
property) must be considered as if scrolled to their origin position.
Note that the baseline of a cell may end up below its bottom border,
see the example below.
The maximum distance between the top of the cell box and the baseline
over all cells that have 'vertical-align: baseline' is used to set the
baseline of the row. Here is an example:
Example of vertically aligning the cells [D]
Diagram showing the effect of various values of 'vertical-align' on
table cells.
Cell boxes 1 and 2 are aligned at their baselines. Cell box 2 has the
largest height above the baseline, so that determines the baseline of
the row.
If a row has no cell box aligned to its baseline, the baseline of that
row is the bottom content edge of the lowest cell in the row.
To avoid ambiguous situations, the alignment of cells proceeds in the
following order:
1. First the cells that are aligned on their baseline are positioned.
This will establish the baseline of the row. Next the cells with
'vertical-align: top' are positioned.
2. The row now has a top, possibly a baseline, and a provisional
height, which is the distance from the top to the lowest bottom of
the cells positioned so far. (See conditions on the cell padding
below.)
3. If any of the remaining cells, those aligned at the bottom or the
middle, have a height that is larger than the current height of the
row, the height of the row will be increased to the maximum of
those cells, by lowering the bottom.
4. Finally the remaining cells are positioned.
Cell boxes that are smaller than the height of the row receive extra
top or bottom padding.
The cell in this example has a baseline below its bottom border:
div { height: 0; overflow: hidden; }
Test
17.5.4 Horizontal alignment in a column
The horizontal alignment of a cell's inline content within a cell box
can be specified by the value of the 'text-align' property on the cell.
17.5.5 Dynamic row and column effects
The 'visibility' property takes the value 'collapse' for row, row
group, column, and column group elements. This value causes the entire
row or column to be removed from the display, and the space normally
taken up by the row or column to be made available for other content.
Contents of spanned rows and columns that intersect the collapsed
column or row are clipped. The suppression of the row or column,
however, does not otherwise affect the layout of the table. This allows
dynamic effects to remove table rows or columns without forcing a
re-layout of the table in order to account for the potential change in
column constraints.
17.6 Borders
There are two distinct models for setting borders on table cells in
CSS. One is most suitable for so-called separated borders around
individual cells, the other is suitable for borders that are continuous
from one end of the table to the other. Many border styles can be
achieved with either model, so it is often a matter of taste which one
is used.
'border-collapse'
Value: collapse | separate | inherit
Initial: separate
Applies to: 'table' and 'inline-table' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property selects a table's border model. The value 'separate'
selects the separated borders border model. The value 'collapse'
selects the collapsing borders model. The models are described below.
17.6.1 The separated borders model
'border-spacing'
Value: ? | inherit
Initial: 0
Applies to: 'table' and 'inline-table' elements*
Inherited: yes
Percentages: N/A
Media: visual
Computed value: two absolute lengths
*) Note: user agents may also apply the 'border-spacing' property to
'frameset' elements. Which elements are 'frameset' elements is not
defined by this specification and is up to the document language. For
example, HTML4 defines a