CSS Values and Units Module Level 3

W3C Candidate Recommendation 30 July 2013

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This CSS3 module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document was produced by the CSS Working Group as a Candidate Recommendation.

A Candidate Recommendation is a document that has been widely reviewed and is ready for implementation. W3C encourages everybody to implement this specification and return comments to the (archived) public mailing list www-style@w3.org (see instructions). When sending e-mail, please put the text “css3-values” in the subject, preferably like this: “[css3-values] …summary of comment…

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

All features described in this specification that also exist in CSS 2.1 [CSS21] are intended to be backwards compatible. If you notice a conflict between this draft and CSS 2.1 [CSS21], please inform the editors!

The CR period will last at least until 29 August 2013. At the time of publication, no test suite and implementation report have yet been made. They will be made available from the CSS test suites page. See the section “CR exit criteria” for details.

See the section “Changes” for changes made to this specification since the last Candidate Recommendation.

The following features are at-risk and may be dropped during the CR period: ‘calc()’, ‘toggle()’, ‘attr()’.

Table of contents

1. Introduction

The value definition field of each CSS property can contain keywords, data types (which appear between ‘<’ and ‘>’), and information on how they can be combined. Generic data types (<length> being the most widely used) that can be used by many properties are described in this specification, while more specific data types (e.g., <spacing-limit>) are described in the corresponding modules.

1.1. Module Interactions

This module replaces and extends the data type definitions in [CSS21] sections, 4.3, and A.2.

2. Value Definition Syntax

The syntax described here is used to define the set of valid values for CSS properties. A property value can have one or more components.

2.1. Component value types

Component value types are designated in several ways:

  1. keyword values (such as ‘auto’, ‘disc’, etc.), which appear literally, without quotes (e.g. auto)
  2. basic data types, which appear between ‘<’ and ‘>’ (e.g., <length>, <percentage>, etc.).
  3. types that have the same range of values as a property bearing the same name (e.g., <‘border-width’> <‘background-attachment’>, etc.). In this case, the type name is the property name (complete with quotes) between the brackets. Such a type does not include CSS-wide keywords such as ‘inherit’.
  4. non-terminals that do not share the same name as a property. In this case, the non-terminal name appears between ‘<’ and ‘>’, as in <spacing-limit>. Notice the distinction between <border-width> and <‘border-width’>: the latter is defined as the value of the ‘border-width’ property, the former requires an explicit expansion elsewhere. The definition of a non-terminal is located near its first appearance in the specification.

Some property value definitions also include the slash (/) and/or the comma (,) as literals. These represent their corresponding tokens.

All CSS properties also accept the CSS-wide keyword values as the sole component of their property value. For readability these are not listed explicitly in the property value syntax definitions. For example, the full value definition of border-color is ‘[ <color>{1,4} ] | inherit | initial’ (even though it is listed as ‘<color>{1,4}’).

Note: This implies that, in general, combining these keywords with other component values in the same declaration results in an invalid declaration. For example, ‘background: url(corner.png) no-repeat, inherit;’ is invalid.

2.2. Component value combinators

Component values can be arranged into property values as follows:

Juxtaposition is stronger than the double ampersand, the double ampersand is stronger than the double bar, and the double bar is stronger than the bar. Thus, the following lines are equivalent:

  a b   |   c ||   d &&   e f
[ a b ] | [ c || [ d && [ e f ]]]

2.3. Component value multipliers

Every type, keyword, or bracketed group may be followed by one of the following modifiers:

For repeated component values (indicated by ‘*’, ‘+’, or ‘#’), UAs must support at least 20 repetitions of the component. If a property value contains more than the supported number of repetitions, the declaration must be ignored as if it were invalid.

2.4. Component values and white space

Unless otherwise specified, white space and/or comments may appear before, after, and/or between components in a property value or subcomponents in a functional notation that is defined using these component combinators and multipliers.

Note: In many cases, spaces will in fact be required between components in order to distinguish them from each other. For example, the value ‘1em2em’ would be parsed as a single DIMEN token with the number ‘1’ and the identifier ‘em2em’, which is an invalid unit. In this case, a space would be required before the ‘2’ to get this parsed as the two lengths ‘1em’ and ‘2em’.

2.5. Property value examples

Below are some examples of properties with their corresponding value definition fields

Property Value definition field Example value
orphans<integer> 3
text-alignleft | right | center | justify center
padding-top<length> | <percentage> 5%
outline-color<color> | invert #fefefe
text-decorationnone | underline || overline || line-through || blink overline underline
font-family[ <family-name> | <generic-family> ]# "Gill Sans", Futura, sans-serif
border-width[ <length> | thick | medium | thin ]{1,4} 2px medium 4px
text-shadow[ inset? && [ <length>{2,4} && <color>? ] ]# | none 3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset
<frequency> && absolute |
[[x-low | low | medium | high | x-high] ||
 [<frequency> | <semitones> | <percentage>]]
-2st x-low

3. Textual Data Types

An identifier is a sequence of characters conforming to the IDENT token in the grammar. [CSS21] Identifiers cannot be quoted; otherwise they would be interpreted as a string.

3.1. Pre-defined Keywords

In the value definition fields, keywords with a pre-defined meaning appear literally. Keywords are CSS identifiers and are interpreted case-insensitively within the ASCII range (i.e., [a-z] and [A-Z] are equivalent).

For example, here is the value definition for the ‘border-collapse’ property:

Value: collapse | separate

And here is an example of its use:

table { border-collapse: separate }

3.1.1. CSS-wide keywords: ‘initial’ and ‘inherit

As defined above, all properties accept the CSS-wide keywords, which represent value computations common to all CSS properties.

The inherit keyword is defined in [CSS21].

The initial keyword represents the specified value that is designated as the property's initial value. [CSS3CASCADE]

Other CSS specifications can define additional CSS-wide keywords. For example, [CSS3CASCADE] adds an ‘unset’ keyword to this set.

3.2. Author-defined Identifiers: the ‘<custom-ident>’ type

Some properties accept arbitrary author-defined identifiers as a component value. This generic data type is denoted by <custom-ident>, and represents any valid CSS identifier that does not otherwise appear as a pre-defined keyword in that property's value definition. Such identifiers are fully case-sensitive, even in the ASCII range (e.g. ‘example’ and ‘EXAMPLE’ are two different, unrelated user-defined identifiers).

The CSS-wide keywords are not valid <custom-ident>s. The ‘default’ keyword is reserved and is also not a valid <custom-ident>s.

Reserved keywords are reserved in all ASCII case permutations.

3.3. Quoted Strings: the ‘<string>’ type

Strings are denoted by <string> and consist of a sequence of characters delimited by double quotes or single quotes. They correspond to the STRING token in the grammar. [CSS21]

Double quotes cannot occur inside double quotes, unless escaped (as "\"" or as "\22"). Analogously for single quotes ('\'' or '\27').

content: "this is a 'string'.";
content: "this is a \"string\".";
content: 'this is a "string".';
content: 'this is a \'string\'.';

It is possible to break strings over several lines, for aesthetic or other reasons, but in such a case the newline itself has to be escaped with a backslash (\). The newline is subsequently removed from the string. For instance, the following two selectors are exactly the same:


a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}

Since a string cannot directly represent a newline, to include a newline in a string, use the escape "\A". (Hexadecimal A is the line feed character in Unicode (U+000A), but represents the generic notion of "newline" in CSS.)

3.4. Resource Locators: the ‘<url>’ type

A URL is a pointer to a resource and is a specially-parsed functional notation denoted by <url>. It corresponds to the URI token in the grammar. [CSS21]

Below is an example of a URL being used as a background image:

body { background: url("http://www.example.com/pinkish.gif") }

The same example can be written without quotes:

body { background: url(http://www.example.com/pinkish.gif) }

Note that in some CSS syntactic contexts (as defined by that context), a URL can be represented as a <string> rather than by <URL>. An example of this is the @import’ rule.

Parentheses, whitespace characters, single quotes (') and double quotes (") appearing in a URL must be escaped with a backslash so that the resulting value is a valid URL token, e.g. ‘url(open\(parens)’, ‘url(close\)parens)’. Depending on the type of URL, it might also be possible to write these characters as URI-escapes (e.g. ‘url(open%28parens)’ or ‘url(close%29parens)’) as described in [URI]. Alternatively a URL containing such characters may be represented as a quoted string within the ‘url()’ notation.

In order to create modular style sheets that are not dependent on the absolute location of a resource, authors should use relative URIs. Relative URIs (as defined in [URI]) are resolved to full URIs using a base URI. RFC 3986, section 3, 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.

When a <url> appears in the computed value of a property, it is resolved to an absolute URL, as described in the preceding paragraph. The computed value of a URI that the UA cannot resolve to an absolute URI is the specified value.

For example, suppose the following rule:

body { background: url("tile.png") }

is located in a style sheet designated by the URL:


The background of the source document's <body> will be tiled with whatever image is described by the resource designated by the URL:


The same image will be used regardless of the URL of the source document containing the <body>.

4. Numeric Data Types

Properties may restrict numeric values to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

CSS theoretically supports infinite precision and infinite ranges for all value types; however in reality implementations have finite capacity. UAs should support reasonably useful ranges and precisions.

4.1. Integers: the ‘<integer>’ type

Integer values are denoted by <integer>. An integer is one or more decimal digits ‘0’ through ‘9’ and corresponds to a subset of the NUMBER token in the grammar. The first digit of an integer may be immediately preceded by ‘-’ or ‘+’ to indicate the integer's sign.

4.2. Numbers: the ‘<number>’ type

Number values are denoted by <number>. A number is either an <integer> or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits. It corresponds to the NUMBER token in the grammar. As with integers, the first character of a number may be immediately preceded by ‘-’ or ‘+’ to indicate the number's sign.

4.3. Percentages: the ‘<percentage>’ type

A percentage value is denoted by <percentage>, consists of a <number> immediately followed by a percent sign ‘%’. It corresponds to the PERCENTAGE token in the grammar.

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.

5. Distance Units: the ‘<length>’ type

Lengths refer to distance measurements and are denoted by <length> in the property definitions. A length is a dimension. However, for zero lengths the unit identifier is optional (i.e. can be syntactically represented as the <number>0’).

A dimension is a number immediately followed by a unit identifier. It corresponds to the DIMENSION token in the grammar. [CSS21] Like keywords, unit identifiers are case-insensitive within the ASCII range.

Properties may restrict the length value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

While some properties allow negative length values, this may complicate the formatting and there may be implementation-specific limits. If a negative length value is allowed but cannot be supported, it must be converted to the nearest value that can be supported.

In cases where the used length cannot be supported, user agents must approximate it in the actual value.

There are two types of length units: relative and absolute.

5.1. Relative lengths

Relative length units specify a length relative to another length. Style sheets that use relative units can more easily scale from one output environment to another.

The relative units are:

Informative Summary of Relative Units
unit relative to
emfont size of the element
exx-height of the element's font
chwidth of the "0" (ZERO, U+0030) glyph in the element's font
remfont size of the root element
vw1% of viewport's width
vh1% viewport's height
vmin1% of viewport's smaller dimension
vmax1% of viewport's larger dimension

Child elements do not inherit the relative values as specified for their parent; they inherit the computed values.

5.1.1. Font-relative lengths: the ‘em’, ‘ex’, ‘ch’, ‘rem’ units

Aside from ‘rem’ (which refers to the font-size of the root element), the font-relative lengths refer to the font metrics of the element on which they are used. The exception is when they occur in the value of the ‘font-size’ property itself, in which case they refer to the computed font metrics of the parent element (or the computed font metrics corresponding to the initial values of the ‘font’ property, if the element has no parent).

em unit

Equal to the computed value of the ‘font-size’ property of the element on which it is used.

The rule:

h1 { line-height: 1.2em }

means that the line height of h1 elements will be 20% greater than the font size of h1 element. On the other hand:

h1 { font-size: 1.2em }

means that the font size of h1 elements will be 20% greater than the computed font size inherited by h1 elements.

ex unit

Equal to the used x-height of the first available font. [CSS3-FONTS] 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 do not 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 heuristic 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 must be assumed.

ch unit

Equal to the used advance measure of the "0" (ZERO, U+0030) glyph found in the font used to render it.

rem unit

Equal to the computed value of ‘font-size’ on the root element.

When specified on the ‘font-size’ property of the root element, the ‘rem’ units refer to the property's initial value.

5.1.2. Viewport-percentage lengths: the ‘vw’, ‘vh’, ‘vmin’, ‘vmax’ units

The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of ‘overflow’ on the root element is ‘auto’, any scroll bars are assumed not to exist. Note that the initial containing block's size is affected by the presence of scrollbars on the viewport.

For paged media, the exact definition of the viewport-percentage lengths is deferred to [CSS3PAGE].

vw unit
Equal to 1% of the width of the initial containing block.

In the example below, if the width of the viewport is 200mm, the font size of h1 elements will be 16mm (i.e. (8×200mm)/100).

h1 { font-size: 8vw }
vh unit
Equal to 1% of the height of the initial containing block.
vmin unit
Equal to the smaller of ‘vw’ or ‘vh’.
vmax unit
Equal to the larger of ‘vw’ or ‘vh’.

5.2. Absolute lengths: the ‘cm’, ‘mm’, ‘in’, ‘pt’, ‘pc’, ‘px’ units

The absolute length units are fixed in relation to each other and anchored to some physical measurement. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc) and the px unit:

unit definition
ininches; 1in is equal to 2.54cm
pxpixels; 1px is equal to 1/96th of 1in
ptpoints; 1pt is equal to 1/72nd of 1in
pcpicas; 1pc is equal to 12pt
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 */
p  { font-size: 12px }    /* px */

For a CSS device, these dimensions are either anchored (i) by relating the physical units to their physical measurements, or (ii) by relating the pixel unit to the reference pixel. For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel.

Note that if the anchor unit is the pixel unit, the physical units might not match their physical measurements. Alternatively if the anchor unit is a physical unit, the pixel unit might not map to a whole number of device pixels.

Note that this definition of the pixel unit and the physical units differs from previous versions of CSS. In particular, in previous versions of CSS the pixel unit and the physical units were not related by a fixed ratio: the physical units were always tied to their physical measurements while the pixel unit would vary to most closely match the reference pixel. (This change was made because too much existing content relies on the assumption of 96dpi, and breaking that assumption breaks the content.)

The reference pixel is 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).

The image below illustrates the effect of viewing distance on the size of a reference pixel: a reading distance of 71 cm (28 inches) results in a reference pixel of 0.26 mm, while a reading distance of 3.5 m (12 feet) results in a reference pixel of 1.3 mm.

This diagram illustrates how the definition of a pixel 		             depends on the users distance from the viewing surface 		             (paper or screen). 		             The image depicts the user looking at two planes, one 		             28 inches (71 cm) from the user, the second 140 inches 		             (3.5 m) from the user. An expanding cone is projected 		             from the user's eye onto each plane. Where the cone 		             strikes the first plane, the projected pixel is 0.26 mm 		             high. Where the cone strikes the second plane, the 		             projected pixel is 1.4 mm high.

Showing that pixels must become larger if the viewing distance increases

This second image illustrates the effect of a device's resolution on the pixel unit: an area of 1px by 1px is covered by a single dot in a low-resolution device (e.g. a typical computer display), while the same area is covered by 16 dots in a higher resolution device (such as a printer).

This diagram illustrates the relationship between the 		             reference pixel and device pixels (called "dots" below). 		             The image depicts a high resolution (large dot density) 		             laser printer output on the left and a low resolution 		             monitor screen on the right. For the laser printer, one 		             square reference pixel is implemented by 16 dots. For 		             the monitor screen, one square reference pixel is 		             implemented by a single dot.

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

6. Other Units

6.1. Angles: the ‘<angle>’ type and ‘deg’, ‘grad’, ‘rad’, ‘turn’ units

Angle values are dimensions denoted by <angle>. The angle unit identifiers are:

Degrees. There are 360 degrees in a full circle.
Gradians, also known as "gons" or "grades". There are 400 gradians in a full circle.
Radians. There are 2π radians in a full circle.
Turns. There is 1 turn in a full circle.

For example, a right angle is ‘90deg’ or ‘100grad’ or ‘0.25turn’ or approximately ‘1.570796326794897rad’.

6.2. Times: the ‘<time>’ type and ‘s’, ‘ms’ units

Time values are dimensions denoted by <time>. The time unit identifiers are:

Milliseconds. There are 1000 milliseconds in a second.

Properties may restrict the time value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

6.3. Frequencies: the ‘<frequency>’ type and ‘Hz’, ‘kHz’ units

Frequency values are dimensions denoted by <frequency>. The frequency unit identifiers are:

Hertz. It represents the number of occurrences per second.
KiloHertz. A kiloHertz is 1000 Hertz.

For example, when representing sound pitches, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a treble sound.

6.4. Resolutions: the <resolution> type and ‘dpi’, ‘dpcm’, ‘dppx’ units

Resolution units are dimensions denoted by <resolution>. The resolution unit identifiers are:

dots per inch
dots per centimeter
dots per ‘px’ unit

The <resolution> unit represents the size of a single "dot" in a graphical representation by indicating how many of these dots fit in a CSS ‘in’, ‘cm’, or ‘px’. For uses, see e.g. the ‘resolution’ media query in [MEDIAQ] or the ‘image-resolution’ property defined in [CSS3-IMAGES].

Note that due to the 1:96 fixed ratio of CSS ‘in’ to CSS ‘px’, ‘1dppx’ is equivalent to ‘96dpi’. This corresponds to the default resolution of images displayed in CSS: see ‘image-resolution’.

The following @media rule uses Media Queries [MEDIAQ] to assign some special style rules to devices that use two or more device pixels per CSS ‘px’ unit:

@media (min-resolution: 2dppx) { ... }

7. Data Types Defined Elsewhere

Some data types are defined in their own modules. The two common ones are <color> and <image>.

7.1. Colors: the ‘<color>’ type

The <color> data type is defined in [CSS21] and extended in [CSS3COLOR]. UAs that support CSS Color Level 3 or its successor must interpret <color> as defined therein.

7.2. Images: the ‘<image>’ type

The <image> data type is defined herein as equivalent to <url>. It is extended in [CSS3-IMAGES]: UAs that support CSS Image Values Level 3 or its successor must interpret <image> as defined therein.

7.3. 2D Positioning: the ‘<position>’ type

The <position> data type is defined herein as equivalent to the <‘background-position’> syntax defined in [CSS21]. It specifies the position of a object area (e.g. background image) inside a positioning area (e.g. background positioning area). It is extended in [CSS3BG]: UAs that support CSS Backgrounds & Borders Level 3 or its successor must interpret <position> as defined therein.

8. Functional Notations

A functional notation is a type of component value that can represent more complex types or invoke special processing. The syntax starts with the name of the function immediately followed by a left parenthesis (i.e. a FUNCTION token) followed by the argument(s) to the notation followed by a right parenthesis. White space is allowed, but optional, immediately inside the parentheses. If a function takes a list of arguments, the arguments are separated by a comma (‘,’) with optional whitespace before and after the comma.

background: url(http://www.example.org/image);
color: rgb(100, 200, 50 );
content: counter(list-item) ". ";
width: calc(50% - 2em);

8.1. Mathematical Expressions: ‘calc()

The calc() function allows mathematical expressions with addition (‘+’), subtraction (‘-’), multiplication (‘*’), and division (‘/’) to be used as component values. The ‘calc()’ expression represents the result of the mathematical calculation it contains, using standard operator precedence rules. It can be used wherever <length>, <frequency>, <angle>, <time>, <number>, or <integer> values are allowed. Components of a ‘calc()’ expression can be literal values, ‘attr()’ or ‘calc()’ expressions, or <percentage> values that resolve to one of the preceding types.

section {
  float: left;
  margin: 1em; border: solid 1px;
  width: calc(100%/3 - 2*1em - 2*1px);
p {
  margin: calc(1rem - 2px) calc(1rem - 1px);

The following sets the ‘font-size’ so that exactly 40em fits within the viewport, ensuring that roughly the same amount of text always fills the screen no matter the screen size.

:root {
	font-size: calc(100vw / 40);

If the rest of the design is specified using the ‘rem’ unit, the entire layout will scale to match the viewport width.

The following example stacks two centered background images, with one offset slightly from the other.

.foo {
	background: url(top.png), url(bottom.png);
	background-repeat: no-repeat;
	background-position: calc(50% + 20px) calc(50% + 20px), 50% 50%;

This example shows how to place color-stops on a gradient an equal distance from either end.

.foo {
	background-image: linear-gradient(to right, silver,
	                                            white 50px,
	                                            white calc(100% - 50px),

8.1.1. Syntax

The expression language of these functions is described by the grammar and prose below.

math    : calc S*;
calc    : "calc(" S* sum S* ")";
sum     : product [ S+ [ "+" | "-" ] S+ product ]*;
product : unit [ S* [ "*" S* unit | "/" S* NUMBER ] ]*;
attr    : "attr(" S* qname [ S+ type-keyword ]? S* [ "," [ unit | calc ] S* ]? ")";
unit    : [ NUMBER | DIMENSION | PERCENTAGE | "(" S* sum S* ")" | calc | attr ];

Note that the grammar requires spaces around binary ‘+’ and ‘-’ operators. The ‘*’ and ‘/’ operators do not require spaces.

Additionally, the following redefinition is made to the informative grammar appearing in CSS 2.1 Appendix G:

  : unary_operator?
      TIME S* | FREQ S* ]
  | STRING S* | IDENT S* | URI S* | hexcolor | function | math

UAs must support ‘calc()’ expressions of at least 20 terms, where each NUMBER, DIMENSION, or PERCENTAGE is a term. If a ‘calc()’ expression contains more than the supported number of terms, it must be treated as if it were invalid.

8.1.2. Type Checking

A math expression has a resolved type, which is one of <length>, <frequency>, <angle>, <time>, <number>, or <integer>. The resolved type must be valid for where the expression is placed; otherwise, the expression is invalid. The resolved type of the expression is determined by the types of the values it contains. NUMBER tokens are of type <number> or <integer>. A DIMENSION token's type is given by its unit (‘cm’ is <length>, ‘deg’ is <angle>, etc.). An ‘attr()’ expression's type is given by its <type-or-unit> argument. If percentages are accepted in the context in which the expression is placed, a PERCENTAGE token has the type of the value that percentages are relative to; otherwise, a math expression containing percentages is invalid.

Operators form sub-expressions, which gain types based on their arguments. To make expressions simpler, operators have restrictions on the types they accept. At each operator, the types of the left and right argument are checked for these restrictions. If compatible, the type resolves as described below (the following ignores precedence rules on the operators for simplicity):

If an operator does not pass the above checks, the expression is invalid. Also, division by zero is invalid. This includes both dividing by the literal number zero, as well as any numeric expression that evaluates to zero (as purely-numeric expressions can be evaluated without any additional information at parse time).

Note that algebraic simplifications do not affect the validity of the ‘calc()’ expression or its resolved type. For example, ‘calc(5px - 5px + 10s)’ or ‘calc(0 * 5px + 10s)’ are both invalid due to the attempt to add a length and a time.

8.1.3. Computed Value

The computed value of a ‘calc()’ expression is the expression with all components computed, with all multiplication/division subexpressions resolved, and with all addition/subtraction subexpressions resolved where both sides are the same type.

Where percentages are not resolved at computed value, they are not resolved in ‘calc()’ expressions, e.g. ‘calc(100% - 100% + 1em)’ resolves to ‘calc(0% + 1em)’, not to ‘calc(1em)’. If there are special rules for computing percentages in a value (e.g. the ‘height’ property), they apply whenever a ‘calc()’ expression contains percentages.

Thus, the computed value of a ‘calc()’ expression can be represented as either a number or a tuple of a dimension and a percentage.

Given the complexities of width and height calculations on table cells and table elements, math expressions involving percentages for widths and heights on table columns, table column groups, table rows, table row groups, and table cells in both auto and fixed layout tables MAY be treated as if ‘auto’ had been specified.

8.1.4. Range Checking

The value resulting from an expression must be clamped to the range allowed in the target context.

Note this requires all contexts accepting ‘calc()’ to define their allowable values as a closed (not open) interval.

These two are equivalent to ‘width: 0px’ since widths smaller than 0px are not allowed.

width: calc(5px - 10px);
width: 0px;

8.2. Toggling Between Values: ‘toggle()

The toggle() expression allows descendant elements to cycle over a list of values instead of inheriting the same value.

The following example makes <em> elements italic in general, but makes them normal if they're inside something that's italic:

em { font-style: toggle(italic, normal); }

The syntax of the ‘toggle()’ expression is:

toggle( <value># )

where <value> is any CSS value that is valid where the expression is placed, and that doesn't contain any top-level commas. If any of the values inside are not valid, then the entire ‘toggle()’ expression is invalid. The ‘toggle()’ expression may be used as the value of any property (but must be the only component in that property's value).

Note that because toggled values are separated by commas, they cannot themselves include top-level commas.

The ‘toggle()’ notation is not allowed to be nested; nor may it contain ‘attr()’ or ‘calc()’ notations. Declarations containing such constructs are invalid.

The value represented by ‘toggle()’ is determined by comparing the inherited value I (i.e. the computed value on the parent, or, for the root, the initial value) to the computed values Cn returned by the n-th argument to ‘toggle()’. For the earliest Cn such that Cn = I, the value returned by toggle is Cn+1. However, if this Cn is the last value, or if there are no Cn that equal I, the computed value of the first value is returned instead.

The following example cycles markers for nested lists, so that a top level list has disc-shaped markers, but nested lists use circle, then square, then box, and then repeat through the list of marker shapes, starting again (for the 5th list deep) with disc.

  ul { list-style-type: disc; }
  ul ul { list-style-type: toggle(disc, circle, square, box); }

Note that ‘toggle()’ explicitly looks at the computed value of the parent, so it works even on non-inherited properties. This is similar to the ‘inherit’ keyword, which is works even on non-inherited properties.

Note that the computed value of a property is an abstract set of values, not a particular serialization [CSS21], so comparison between computed values should always be unambiguous and have the expected result. For example, a Level 2 background-position computed value is just two offsets, each represented as an absolute length or a percentage, so the declarations ‘background-position: top center’ and ‘background-position: 50% 0%’ produce identical computed values. If the "Computed Value" line of a property definition seems to define something ambiguous or overly strict, please provide feedback so we can fix it.

8.3. Attribute References: ‘attr()

The attr() function is allowed as a component value in properties applied to an element or pseudo-element. It returns the value of an attribute on the element. If used on a pseudo-element, it returns the value of the attribute on the pseudo-element's originating element.

The computed value of the ‘attr()’ expression is the value of the attribute with the specified name on the element, according to the rules given below.

In CSS2.1 [CSS21], the ‘attr()’ expression always returns a string. In CSS3, the ‘attr()’ expression can return many different types. The ‘attr()’ expression cannot return everything, for example it cannot do counters, named strings, quotes, or keyword values such as ‘auto’, ‘nowrap’, or ‘baseline’. This is intentional, as the intent of the ‘attr()’ expression is not to make it possible to describe a presentational language's formatting using CSS, but to enable CSS to take semantic data into account.

The new syntax for the ‘attr()’ expression is:

attr( <attr-name> <type-or-unit>? [ , <fallback> ]? )

where <attr-name> is a CSS qualified name (the qname production in [CSS3NAMESPACE]) that represents an attribute name. (In the absence of namespacing, this will just be a CSS identifier.)

The optional <type-or-unit> argument is a keyword drawn from the list below that tells the UA how to interpret the attribute value, and defines a type for the attr() expression. If omitted, ‘string’ is implied.

The optional <fallback> argument represents a fallback value, which is used if the named attribute is missing, or its value cannot be parsed into the given type or is invalid/out-of-range for the property. If it's absent, the default value for the given <type-or-unit> (from the list below) is implied.

Note that, unlike ‘toggle()<value>s, an ‘attr()<fallback> value may contain top-level commas, as it is always the last argument in the functional notation.

The attr() expression is only valid if:

Note that the default value need not be of the type given, if the attr() expression is the entire property value. For instance, if the type required of the attribute by the author is ‘px’, the default could still be ‘auto’, like in ‘width: attr(size px, auto);’.

If the attr() is used alongside other values to form the full property value, however, the default value must match the attr()'s type. For example, ‘box-shadow: attr(size px, inset) 5px 10px blue;’ is invalid, even though it would create a valid declaration if you substituted the attr() expression with either a ‘px’ length or the ‘inset’ keyword.

If the specified attribute exists on the element, the value of the attribute must be parsed as required by the <type-or-unit> argument (as defined in the list below). Unless the type is ‘string’, it must first be stripped of leading and trailing white space. The resulting value is the attr() expression's value. If the value did not parse as required, the attr() expression's value is its fallback value.

The <type-or-unit> keywords are:

The attribute value is taken as the contents of a CSS <string>. The default is the empty string.

Note, this does not reparse the attribute value with the CSS parser. So, for example, an attribute whose value is "\33" will produce a string containing those three characters, not a string containing "3" (the character that the escape would evaluate to).

The attribute value must parse as a HASH or IDENT CSS token, and be successfully interpreted as a <color>. The default is ‘currentColor’.
The attribute value is taken as the contents of a CSS <string>. It is interpreted as a quoted string within the ‘url()’ notation. The default is ‘about:invalid’, which is a URI defined (in Appendix A) to point to a non-existent document with a generic error condition. Relative URLs must be made absolute according to the rules of the document language as applied to URLs originating from the element; they are not relative to the style sheet.
The attribute value must parse as a NUMBER CSS token, and be successfully interpreted as an <integer>. The default is ‘0’, or else the property's minimum value if ‘0’ is not valid for the property. The default must also be used if the property in question only accepts integers within a certain range and the attribute is out of range.
The attribute value must parse as a NUMBER CSS token, and is interpreted as an <number>. The default is ‘0’, or else the property's minimum value if ‘0’ is not valid for the property. The default must also be used if the property in question only accepts integers within a certain range and the attribute is out of range.
The attribute value must parse as a DIMENSION CSS token, and be successfully interpreted as the specified type. The default is ‘0’ in the relevant units, or else the property's minimum value if ‘0’ in the relevant units is not valid for the property. The default must also be used if the property in question only accepts values within a certain range (e.g. positive lengths or angles from 0 to 90deg) and the attribute is out of range (e.g. a negative length or 180deg). If the unit is a relative length, it must be computed to an absolute length.
The attribute value must parse as a NUMBER CSS token, and is interpreted as a dimension with the specified unit. The default is ‘0’ in the relevant units, or else the property's minimum value if ‘0’ in the relevant units is not valid for the property. The default must also be used if the property in question only accepts values within a certain range (e.g. positive lengths or angles from 0 to 90deg) and the attribute is out of range (e.g. a negative length or 180deg). If the unit is a relative length, it must be computed to an absolute length.

This example shows the use of attr() to visually illustrate data in an XML file:

	<wood length="12"/>
	<wood length="5"/>
	<metal length="19"/>
	<wood length="4"/>

stock::before {
	display: block;
	content: "To scale, the lengths of materials in stock are:";
stock > * {
	display: block;
	width: attr(length em); /* default 0 */
	height: 1em;
	border: solid thin;
	margin: 0.5em;
wood {
	background: orange url(wood.png);
metal {
	background: silver url(metal.png);

All of the following examples are invalid and would cause a parse-time error, and thus cause the relevant declaration—in this case all of them—to be ignored:

content: attr(title color); /* 'content' doesn't accept colors */

content: attr(end-of-quote string, inherit) close-quote;
/* the 'inherit' value is not allowed there, since the result would be
'inherit close-quote', which is invalid. */

margin: attr(vertical length) attr(horizontal deg);
/* deg units are not valid at that point */

color: attr(color); /* 'color' doesn't accept strings */

The ‘attr()’ expression cannot currently fall back onto another attribute. Future versions of CSS may extend ‘attr()’ in this direction.

9. Appendix A: IANA Considerations

9.1. Registration for the about:invalid URL scheme

This sections defines and registers the about:invalid URI, in accordance with the registration procedure defined in the about: uri scheme RFC draft.

The official record of this registration can be found at http://www.iana.org/assignments/about-uri-tokens/about-uri-tokens.xml.

Registered Token invalid
Intended Usage

The about:invalid URI references a non-existent document with a generic error condition. It can be used when a URI is necessary, but the default value shouldn't be resolveable as any type of document.

Contact/Change controller CSS WG <www-style@w3.org> (on behalf of W3C)
Specification CSS Values and Units Module Level 3


Comments and suggestions from Giovanni Campagna, Christoph Päper, Keith Rarick, Alex Mogilevsky, Ian Hickson, David Baron, Edward Welbourne, Boris Zbarsky, Björn Höhrmann and Michael Day improved this module.


Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance classes

Conformance to CSS Values and Units Level 3 is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to CSS Values and Units Level 3 if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to CSS Values and Units Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined CSS Values and Units Level 3 by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to CSS Values and Units Level 3 if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.
a user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.


Changes since the 28 August 2012 Candidate Recommendation are:

Changes since the 4 April 2013 Candidate Recommendation are:


Normative references

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
John Daggett. CSS Fonts Module Level 3. 12 February 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-fonts-20130212/
Elika J. Etemad; Tab Atkins Jr. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-images-20120417/
Bert Bos; Elika J. Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 24 July 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-background-20120724/
Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607
Elika J. Etemad; Anne van Kesteren. CSS Namespaces Module. 29 September 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-namespace-20110929/
Melinda Grant; et al. CSS Paged Media Module Level 3. 14 March 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-page-20130314/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
T. Berners-Lee; R. Fielding; L. Masinter. Uniform Resource Identifiers (URI): generic syntax. January 2005. Internet RFC 3986. URL: http://www.ietf.org/rfc/rfc3986.txt

Other references

Håkon Wium Lie; Elika J. Etemad; Tab Atkins Jr. CSS Cascading and Inheritance Level 3. 3 January 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-cascade-20130103/
Florian Rivoal. Media Queries. 19 June 2012. W3C Recommendation. URL: http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/