6 Assigning property values, Cascading, and Inheritance


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 three-stepfour-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 "computed"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 aeach 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, generally the computed value.element.
  3. Otherwise use the property's initial value. The initial value of each property is indicated in the property's definition.

Since it has no parent, the root of the document tree cannot use values from the parent element; in this case, the initial value is used if necessary.6.1.2 Computed values

Specified values may be absolute (i.e., theyare not specified relativeresolved to another value, as in 'red' or '2mm') orcomputed values during the cascade; for example valid relative (i.e., theyURIs are specified relative to another value, as in 'auto', '2em',made absolute and '12%'). For'em' and 'ex' units are computed to pixel or absolute values, no computation is neededlengths. Computing a value never requires the user agent to findrender the computed value. Relative values, ondocument.

The other hand, must be transformed intocomputed values: percentages must be multiplied by a reference value (each property defines whichvalue that is), values with relative units (em, ex, px) must be madeof invalid and absolute by multiplying with the appropriate font or pixel size, 'auto' values must be computed byURIs is the formulas given with each property, certain keywords ('smaller', 'bolder', 'inherit') must be replaced according to their definitions. In most cases, elements inherit computed values. However, there are some properties whosespecified value may be inherited (e.g.,value.

When the numberspecified value for the 'line-height' property). In the cases where child elements dois not inherit'inherit', the computed value, thisvalue of a property is described indetermined as specified by the property definition. 6.1.3 Actual values AComputed Value isline in principle ready to be used, but a user agent may not be able to make usethe definition of the value in a given environment.property. See the section on inheritance for example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximatethe definition of computed width.values when the actualspecified value is 'inherit'.

The computed value after any approximations have been applied. 6.2exists 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 can be relative to each other; for example a width could be set as a percentage, which is dependent on the containing block's width. The used value is the result of taking the computed value and resolving these dependencies into a final absolute value used for the actual layout.

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:

<H1>The headline <EM>is</EM> important!</H1>

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.

To set a "default" style property for a document, authors may set the property on the root of the document tree. In HTML, for example, the HTML or BODYWhen inheritance occurs, elements can serve this function. Note that this will work even if the author omits the BODY tag in the HTML source since the HTML parser will infer the missing tag. Example(s): For example, sinceinherit computed values. The 'color' property is inherited, all descendants ofcomputed value from the BODYparent element will inheritbecomes both the color 'black': BODY { color: black; }specified percentage values are not inherited;value and the computed values are.value on the child.


For example, given the following style sheet:

body { font-size: 10pt }
h1 { font-size:  120%130% }

and this document fragment:

  <H1>A <EM>large</EM> heading</H1>

the 'font-size' property for the H1 element will have the computed value '12pt' (120%'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 '12pt''13pt' as well. If the user agent does not have the 12pt13pt font available, the actual value of 'font-size' for both H1 and EM might be, for example, '11pt'.'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 inherited value, which is normally only used as a fallback value,'inherit' value can be strengthened by setting 'inherit' explicitly.used to strengthen inherited values, and it can also be used on properties that are not normally inherited.


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;transparent !important;

6.3 The @import rule

The '@import' rule allows users to import style rules from other style sheets. Any @import rules must precede all rule sets in a style sheet. 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.


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.


The following rules have the same effect as if the imported style sheet were wrapped in an @media rule for the same media, but it may save the UA a fruitless download.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.

6.4 The cascade

Style sheets may have three different origins: author, user, and user agent.

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 rulesuser and author rules have more weight than rules in the UA's default style sheet.

Imported style sheets also cascade and their weight depends on their import order. Rules specified in a given style sheet override rules imported from other style sheets. Imported style sheets can themselves import and override other style sheets, recursively, and the same precedence rules apply.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.
  2. The primarySort of the declarations isby weightimportance (normal or important) and origin: for normal declarations, author style sheets overrideorigin (author, user, or user agent). In ascending order:
    1. user agent style sheets
    2. which override the default style sheet. For "!important" declarations,user normal style sheets
    3. overrideauthor normal style sheets
    4. which override the default style sheet. "!important" declaration override normal declarations. An importedauthor important style sheet has the same origin as thesheets
    5. user important style sheet that imported it. The secondarysheets
  3. Sort isby 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 rules have the same weight, origin and specificity, the latter specified wins. Rules in imported style sheets are considered to be before any rules 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. It is therefore important that the user agent give the user the ability to turn off the influence of a certain style sheet, e.g., through a pull-down menu.

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 keywordsdelimiter 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.

Note. This is a semantic change since CSS1. In CSS1, author "!important" rules took precedence over user "!important" rules.Declaring a shorthand property (e.g., 'background') to be "!important" is equivalent to declaring all of its sub-properties to be "!important".


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 stylesstyle 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:

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 number of element namesid attribute is defined as an "ID" in the selector (= c) ignore pseudo-elements.source document's DTD.

Concatenating the threefour numbers a-b-ca-b-c-d (in a number system with a large base) gives the specificity.


Some examples:

 *             {}  /* a=0 b=0 c=0 d=0 -> specificity =  00,0,0,0 */
 li            {}  /* a=0 b=0  c=1c=0 d=1 -> specificity =  10,0,0,1 */
 li:first-line {}  /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
 ul li         {}  /* a=0 b=0  c=2c=0 d=2 -> specificity =  20,0,0,2 */
 ul ol+li      {}  /* a=0 b=0  c=3c=0 d=3 -> specificity =  30,0,0,3 */
 h1 + *[rel=up]{}  /* a=0  b=1b=0 c=1 d=1 -> specificity =  110,0,1,1 */
 ul ol li.red  {}  /* a=0  b=1 c=3b=0 c=1 d=3 -> specificity =  130,0,1,3 */
 li.red.level  {}  /* a=0  b=2 c=1b=0 c=2 d=1 -> specificity =  210,0,2,1 */
 #x34y         {}  /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
 style=""          /* a=1 b=0 c=0 d=0 -> specificity =  1001,0,0,0 */

 In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, but for the purpose of step 3 of the cascade algorithm, they are considered to have an ID selector (specificity: a=1, b=0, c=0). For the purpose of step 4, they are considered to be after all other rules.<HEAD>
<STYLE type="text/css">
  #x97z { color:  bluered }
<P ID=x97z style="color:  red">green">

In the above example, the color of the P element would be red. Although the specificity is the same for both declarations,green. The declaration in the "style" attribute will override the one in the STYLE element because of cascading rule 4.3, since it has a higher specificity.

6.4.4 Precedence of non-CSS presentational hints

The UA may choose to honor presentational hints from other sources than style sheets, for example the FONT element or the "align" attributeattributes in HTML.an HTML source document. If so, the non-CSS presentational hints must bethese attributes are translated to the corresponding CSS rules with specificity equal to zero. The rules0, and are assumed to betreated as if they were inserted at the start of the author style sheet andsheet. They may therefore be overridden by subsequent style sheet rules. Note.In a transition phase, this policy will make it easier for stylistic attributes to coexist with style sheets.

Note.For HTML, any attribute that is not in CSS1,the non-CSS presentational hints were given a specificity equal to 1, not 0.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 change is due touser agent style sheet.


The introductionfollowing user stylesheet would override the font weight of 'b' elements in all documents, and the universal selector, which has a specificitycolor 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 0.font elements in all documents:

font[color] { color: orange ! important; }