csswg-drafts/css3-cascade/Overview.html at 8fca1c98513a69bb76124be34cf267d711f6c3a7 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<html lang=en>
 <head><meta content="text/html" http-equiv=Content-Type>

  <title>CSS Cascading and Inheritance Level 3</title>
  <link href="../default.css" rel=stylesheet type="text/css">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
  type="text/css">

  <style>
table { border-collapse: collapse; border: 1px solid black }
th, td { border: 1px solid black; padding: 0.2em; text-align: left }
.issue { color: red }
</style>

 <body>
  <div class=head> <!--begin-logo-->
   <p><a href="http://www.w3.org/"><img alt=W3C height=48
    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1>CSS Cascading and Inheritance Level 3</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 31 October
    2011</h2>

   <dl>
    <dt>This version:

    <dd><a
     href="http://www.w3.org/TR/2011/ED-css3-cascade-20111031/">http://www.w3.org/TR/2011/ED-css3-cascade-20111031/</a>


    <dt>Latest version:

    <dd><a
     href="http://www.w3.org/TR/css3-cascade">http://www.w3.org/TR/css3-cascade</a>


    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2005/WD-css3-cascade-20051215">http://www.w3.org/TR/2005/WD-css3-cascade-20051215</a>


    <dt>Editor:

    <dd>H&aring;kon Wium Lie, Opera Software, howcome@opera.com
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2011 <a
    href="http://www.w3.org/"><acronym
    title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
    href="http://www.csail.mit.edu/"><acronym
    title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
    href="http://www.ercim.eu/"><acronym
    title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
    <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

  <h2 class="no-num no-toc" id=abstract>Abstract</h2>

  <p>This CSS3 module describes how to find the specified value for all
   properties on all elements. Values either come from style sheets or the
   property's initial value. By way of cascading and inheritance, values are
   propagated to all properties on all elements.

  <p>This module interfaces with several other modules. It relies on <a
   href="#CSS3SYN" rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a> to parse
   the style sheets, on <a href="#SELECT"
   rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a> to calculate specificity,
   and on <a href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> to
   determine if a declaration applies to the media. The output from this
   module is a <em>specified value</em> for all element/property
   combinations. The processing of specified values is described in <a
   href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.

  <h2 class="no-num no-toc" id=status-of-this-document>Status of this
   document</h2>

  <p>This is a draft of a <a href="http://www.w3.org/TR/css3-roadmap/">module
   of CSS level 3</a>. It will probably be bundled with some other modules
   before it becomes a <a href="http://www.w3.org/TR/#About">W3C
   Recommendation</a>.

  <p>The main purpose of this module is to rewrite the relevant parts of CSS2
   as a module for CSS3. With the exception of the &lsquo;<code
   class=css>initial</code>&rsquo; value and the optional title for
   &lsquo;<code class=css>@import</code>&rsquo; and &lsquo;<a
   href="#media"><code class=css>@media</code></a>&rsquo;, all features
   described in this module also exist in CSS2. Compared to CSS2, the
   cascading order has been changed in two cases as noted in the text.

  <p>This draft should not be cited except as "work in progress". It is a
   work item of the CSS working group and part of the <a href="/Style/">Style
   activity</a>. It may be modified or dropped altogether at any point in
   time. Implementations for the purpose of experimenting with the
   specification are welcomed, as long as they are clearly marked as
   experimental.

  <p>Feedback on this draft is invited. The preferred place for discussion is
   the (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a href="http://www.w3.org/Mail/Lists.html#www-style"
   lang=und>www-style@w3.org</a>. W3C members can also send comments to the
   CSS WG mailing list.

  <p>A list of current W3C Recommendations and other technical documents
   including Working Drafts and Notes can be found at <a
   href="http://www.w3.org/TR/">http://www.w3.org/TR</a>.

  <h2 class="no-num no-toc" id=table-of-contents>Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>

   <li><a href="#combine"><span class=secno>2. </span>Combining Style
    Sheets</a>
    <ul class=toc>
     <li><a href="#at-import"><span class=secno>2.1. </span>Importing style
      sheets</a>

     <li><a href="#altss"><span class=secno>2.2. </span> Alternate Style
      Sheets</a>

     <li><a href="#at-media-rule"><span class=secno>2.3. </span> Named
      <code>@media</code> rules</a>
    </ul>

   <li><a href="#cascade"><span class=secno>3. </span> Cascading and
    Inheritance</a>
    <ul class=toc>
     <li><a href="#cascading"><span class=secno>3.1. </span> Cascading</a>

     <li><a href="#inheritance"><span class=secno>3.2. </span>
      Inheritance</a>

     <li><a href="#initial"><span class=secno>3.3. </span> Initial value</a>

     <li><a href="#computing-weight"><span class=secno>3.4. </span> Computing
      weight</a>
    </ul>

   <li><a href="#conformance"><span class=secno>4. </span>Conformance</a>

   <li class=no-num><a href="#acknowledgments">Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>
    <ul class=toc>
     <li class=no-num&g
402E
t;<a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>

   <li class=no-num><a href="#property-index">Property index</a>
  </ul>
  <!--end-toc-->

  <hr>

  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <p>One of the fundamental design principles of CSS is <em>cascading</em>,
   which allows several style sheets to influence the presentation of a
   document. When different declarations try to set a value for the same
   element/property combination, the conflicts must somehow be resolved.

  <p>The opposite problem arises when no declarations try to set a the value
   for an element/property combination. In this case, a value must be found
   by way of <a href="#inherited"><em>inheritance</em></a> or by looking at
   the property's <a href="#initial-value"><em>initial value</em></a>.

  <p>The rules for finding a the specified value for all properties on all
   elements is described in this specification. The rules for finding the
   specified values in the page context and the margin boxes are described in
   <a href="#CSS3PAGE" rel=biblioentry>[CSS3PAGE]<!--{{CSS3PAGE}}--></a>.

  <h2 id=combine><span class=secno>2. </span>Combining Style Sheets</h2>

  <h3 id=at-import><span class=secno>2.1. </span>Importing style sheets</h3>

  <p>The <dfn id=import>&lsquo;<code class=css>@import</code>&rsquo;</dfn>
   rule allows users to import style rules from other style sheets. Any
   &lsquo;<code class=css>@import</code>&rsquo; rules must follow all
   &lsquo;<code class=css>@charset</code>&rsquo; rules and precede all other
   at-rules and rule sets in a style sheet. The &lsquo;<code
   class=css>@import</code>&rsquo; 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(&hellip;) around it.

  <div class=example>
   <p>The following lines are equivalent in meaning and illustrate both
    &lsquo;<code class=css>@import</code>&rsquo; syntaxes (one with
    &lsquo;<code class=css>url()</code>&rsquo; and one with a bare string):

   <pre>
@import "mystyle.css";
@import url("mystyle.css");
</pre>
  </div>

  <p>So that user agents can avoid retrieving resources for unsupported media
   types, authors may specify media-dependent @import rules. These <dfn
   id=conditional-imports
   title="conditional import|media-dependent import">conditional
   imports</dfn> specify comma-separated &ldquo;media queries&rdquo; after
   the URI.

  <div class=example>
   <p>The following rules illustrate how &lsquo;<code
    class=css>@import</code>&rsquo; rules can be made media-dependent:

   <pre>
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);
</pre>
  </div>

  <p>The full syntax of the expressions after the URL is defined by the Media
   Queries specification <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>.

  <p>In the absence of any media queries, the import is unconditional.
   Specifying &lsquo;<code class=property>all</code>&rsquo; for the medium
   has the same effect.

  <h3 id=altss><span class=secno>2.2. </span> Alternate Style Sheets</h3>

  <p>At the end of the &lsquo;<code class=css>@import</code>&rsquo; rule
   (after any media queries), there may be a string that assigns a name to
   the import.

  <div class=example>
   <pre>
@import url(layout1.css) screen "Plain style";
@import url(colors1.css) screen "Plain style";
@import url(style4.css) "Four-columns and dark";
</pre>
  </div>

  <p>Imports with different names (together with any named <a
   href="#at-media-rule">@media rules,</a> see below) represent <dfn
   id=alternative-style-sheets>alternative style sheets</dfn>. The UA should
   provide a way for the user to select an alternative by name.

  <p>The rules for which style sheet to import are as follows:

  <ul>
   <li>
    <p>If the user has not chosen a style by name (e.g., he has indicated to
     the UA that he wants the default style), then all &lsquo;<code
     class=css>@import</code>&rsquo;s without a name are imported, as well as
     the first &lsquo;<code class=css>@import</code>&rsquo; that has a name
     and all other &lsquo;<code class=css>@import</code>&rsquo;s that have
     the same name.

   <li>
    <p>If the user has chosen a style be name, then all &lsquo;<code
     class=css>@import</code>&rsquo;s without a name are imported, as well as
     all &lsquo;<code class=css>@import</code>&rsquo;s with the name chosen
     by the user.
  </ul>

  <div class=example>
   <p>If the UA is using the default style, only &ldquo;Yellow Fish&rdquo;
    and the nameless &lsquo;<code class=css>@import</code>&rsquo;s (shown in
    <code><b>bold</b></code>) are used, the others are skipped:

   <pre>
<b>@import "common1.css";</b>
<b>@import "yellowfish.css" "Yellow Fish";</b>
@import "simple.css"     "Simple";
<b>@import "extra.css"      "Yellow Fish";</b>
@import "deco-a.css"     "Budapest";
@import "deco-b.css"     "Budapest";
<b>@import "common2.css";</b>
</pre>
  </div>

  <div class=example>
   <p>If the user has chosen the &ldquo;Simple&rdquo; style, only
    &ldquo;Simple&rdquo; and the nameless &lsquo;<code
    class=css>@import</code>&rsquo;s (all shown in <code><b>bold</b></code>)
    are used, the others are skipped:

   <pre>
<b>@import "common1.css";</b>
@import "yellowfish.css" "Yellow Fish";
<b>@import "simple.css"     "Simple";</b>
@import "extra.css"      "Yellow Fish";
@import "deco-a.css"     "Budapest";
@import "deco-b.css"     "Budapest";
<b>@import "common2.css";</b>
</pre>
  </div>

  <p>Style sheets are only imported if the media query on the &lsquo;<code
   class=css>@import</code>&rsquo; (if any) matches the media for which the
   UA renders the document.

  <p>If an &lsquo;<code class=css>@import</code>&rsquo; is skipped, then all
   &lsquo;<code class=css>@import</code>&rsquo;s inside the style sheet it
   points to are also skipped, no matter what their names.

  <p>The list of alternative style sheets that the user can choose from
   consists of:

  <ul>
   <li>any names in the document source itself (such as <code
    class=html>title</code> attributes on <code class=html>link</code> and
    <code class=html>style</code> elements in HTML and<code
    class=xml>title</code> pseudo-attributes on <code
    class=xml>&lt;?xml-stylesheet?&gt;</code> instructions),

   <li>the names of &lsquo;<a href="#media"><code
    class=css>@media</code></a>&rsquo; and &lsquo;<code
    class=css>@import</code>&rsquo; rules that occur in linked style sheets
    that don't have a name on the link, and

   <li>the names of &lsquo;<a href="#media"><code
    class=css>@media</code></a>&rsquo; and &lsquo;<code
    class=css>@import</code>&rsquo; rules that occur in style sheets linked
    from &lsquo;<code class=css>@import</code>&rsquo; rules that have no
    name, recursively.
  </ul>

  <p>I.e., the UA must not look for additional names by following links with
   a <code class=html>title</code> attribute or named &lsquo;<code
   class=css>@import</code>&rsquo; rules.

  <p class=note>HTML <a href="#HTML401"
   rel=biblioentry>[HTML401]<!--{{HTML401}}--></a> uses the term <dfn
   id=preferred-style-sheet>preferred style sheet</dfn> for the named style
   sheets that the author marked as the default. In CSS, these are all style
   sheets whose name is the first name to occur on any &lsquo;<code
   class=css>@import</code>&rsquo; or &lsquo;<a href="#media"><code
   class=css>@media</code></a>&rsquo;. <dfn
   id=alternate-style-sheets>Alternate style sheets</dfn> are all other named
   style sheets. Finally, HTML calls style sheets without a name <dfn
   id=persistent-style-sheets>persistent style sheets</dfn>, because they are
   imported together with the preferred ones as well as with any
   alternatives.

  <p class=note>The UA should not only provide the user with a choice of
   alternative style sheets, but should also allow the user to turn all style
   sheets off. <!-- See <span class=issue>[where?]</span>-->

  <p class=issue>Are names matched case-sensitively or case-insensitively?
   Property names and font names are case-insensitive, so it is probably most
   consistent to treat style sheet names the same way.

  <p class=issue>Would it be easier to read if we precede the name with a
   keyword or some punctuation? E.g., &lsquo;<code class=css>@import
   url(foo.css) as "Boxed"</code>&rsquo;.

  <p class=issue>[Should the spec state that (1) the media list specified in
   an @import rule prevents the import from being processed if the medium
   doesn't match, but it doesn't "associate" the media with the style sheet
   so that it can't be imported some other way. (2) @import without a medium
   must ignore any medium specified for the same style sheet in a link from a
   document?]

  <h3 id=at-media-rule><span class=secno>2.3. </span> Named <a
   href="#media"><code>@media</code></a> rules</h3>

  <p>An <dfn id=media>@media</dfn> rule specifies the target media types
   (separated by commas) of a set of rules (delimited by curly braces). The
   &lsquo;<a href="#media"><code class=css>@media</code></a>&rsquo; construct
   allows style sheet rules for various media in the same style sheet:

  <div class=example>
   <pre>
@media print {
  body { font-size: 12pt; }
  h1 { font-size: 24pt; }
}
@media screen and (color) {
  body { font-size: medium; }
  h1 { font-size: 2em; }
}
@media screen, print {
  body { line-height: 1.2; }
}
</pre>
  </div>

  <p>The full syntax of the expressions after &lsquo;<a href="#media"><code
   class=css>@media</code></a>&rsquo; is defined by the Media Queries
   specification <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>.

  <p>After the media queries, before the "{", there may be a string that
   assigns a name to the &lsquo;<a href="#media"><code
   class=css>@media</code></a>&rsquo; rule.

  <div class=example>
   <pre>
@media all "Ultra blue" {
  body { background: blue }
  h1 { color: cyan }
}
@media screen, print "Simple and light" {
  body { background: white; color: #333 }
  h2 { font-size: bigger }
}
</pre>
  </div>

  <p>All &lsquo;<a href="#media"><code class=css>@media</code></a>&rsquo;
   rules with a name, together with any &lsquo;<code
   class=css>@import</code>&rsquo; rules with the same name, present
   alternative style sheets. The UA must read the rules inside some &lsquo;<a
   href="#media"><code class=css>@media</code></a>&rsquo; rules and skip
   others, as follows:

  <ul>
   <li>
    <p>If the user has not chosen a style by name, the UA must read all
     &lsquo;<a href="#media"><code class=css>@media</code></a>&rsquo; rules
     of which (1) the media query matches the UA and (2) that either have no
     name or whose name is equal to that of the first named &lsquo;<code
     class=css>@import</code>&rsquo; or the first named &lsquo;<a
     href="#media"><code class=css>@media</code></a>&rsquo;, whichever comes
     first.

   <li>
    <p>If the user has chosen a style by name, the UA must read all &lsquo;<a
     href="#media"><code class=css>@media</code></a>&rsquo; rules (1) of
     which the media query matches the UA and (2) that either have no name or
     whose name is equal to the user's chosen name.
  </ul>

  <div class=example>
   <p>If the user wants the UA to use the default style, the following
    example results in the UA reading the &ldquo;Reverse video&rdquo; style,
    the unnamed &lsquo;<a href="#media"><code
    class=css>@media</code></a>&rsquo; rule and all rules that are not in any
    &lsquo;<a href="#media"><code class=css>@media</code></a>&rsquo; (all
    shown in <code><b>bold</b></code>). We assume the UA uses the
    &lsquo;<code class=css>screen</code>&rsquo; media.

   <pre>
<b>body { background: white }</b>
<b>p { text-indent: 2em }</b>

<b>@media screen {
  h1 { font-size: xx-large }
}</b>
<b>@media screen "Reverse video" {
  body { background: black; color: white }
}</b>
@media screen, print "Rainbow style" {
  p { color: #E0D }
}
</pre>
  </div>

  <div class=example>
   <p>If the user wants the UA to use the &ldquo;Rainbow style&rdquo; style,
    the following example results in the UA reading the &ldquo;Rainbow
    style&rdquo; &lsquo;<a href="#media"><code
    class=css>@media</code></a>&rsquo; rule, the unnamed &lsquo;<a
    href="#media"><code class=css>@media</code></a>&rsquo; rule and all rules
    that are not in any &lsquo;<a href="#media"><code
    class=css>@media</code></a>&rsquo; (all shown in
    <code><b>bold</b></code>). We assume the UA uses the &lsquo;<code
    class=css>screen</code>&rsquo; media.

   <pre>
<b>@import url(base.css);</b>
@import url(altbase.css) "Reverse video";

<b>body { background: white }</b>

<b>@media screen {
  h1 { font-size: xx-large }
}</b>
@media screen "Reverse video" {
  body { background: black; color: white }
}
<b>@media screen, print "Rainbow style" {
  p { color: #E0D }
}</b>
</pre>
  </div>

  <h2 id=cascade><span class=secno>3. </span> Cascading and Inheritance</h2>

  <p>The input to the computations described in this module is:

  <ul>
   <li>a set of parsed style sheets

   <li>the initial value of each property

   <li>the title of the preferred style sheet (if any)
  </ul>

  <p>The output of the computations described in this module is a specified
   value for all properties on all elements.

  <p>In order to find the specified values, implementations must first
   identify which statements that apply to the document. This is done by:

  <ol>
   <li>Starting out with all author style sheets specified in the document,
    as well all user and UA style sheets.

   <li>Removing statements that have associated Media Queries <a
    href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> that are
    false in the given context.

   <li>Removing @import statements that do not match the title of the
    preferred style sheet, if any. This is described in <a
    href="#import">Importing style sheets</a> below.

   <li>Removing @media statements that do not match the title of the
    preferred style sheet, if any. This is described in <a
    href="#import">Named @media rules</a> below.

   <li>Expanding all remaining @import rules.
  </ol>

  <p>Then, for every element, the value for each property can be found by
   following this pseudo-algorithm:

  <ol>
   <li>Identify all declarations that apply to the element (as described in
    <a href="#SELECT" rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>)

   <li>If the cascading process (described below) yields a winning
    declaration and the value of the winning declaration is not &lsquo;<code
    class=css>initial</code>&rsquo; or &lsquo;<code
    class=css>inherit</code>&rsquo;, the value of the winning declaration
    becomes the specified value.

   <li>Otherwise, if the value of the winning declaration is &lsquo;<code
    class=css>inherit</code>&rsquo;, the inherited value (see below) becomes
    the specified value.

   <li>Otherwise, if the value of the winning declaration is &lsquo;<code
    class=css>initial</code>&rsquo;, the initial value (see below) becomes
    the specified value.

   <li>Otherwise, if the property is inherited, the inherited value becomes
    the specified value.

   <li>Otherwise, the initial value becomes the specified value.
  </ol>

  <p>The specified value may need some computation before it can be used.
   This is described in the <a href="/TR/css3-values">Values and Units</a> <a
   href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a> module.

  <h3 id=cascading><span class=secno>3.1. </span> Cascading</h3>

  <p>The purpose of cascading is to find one <dfn
   id=winning-declaration>winning declaration</dfn> among the set of
   declarations that apply for a given element/property combination.

  <p>User agents must sort declarations according to the following criteria,
   in order of importance:

  <ol>
   <li>Weight. In CSS3, the weight of a declaration is based on the origin of
    the declaration and its marked level of importance. See <a
    href="#computing-weight">Computing Weight</a> below for how to compute
    the weight. The declaration with the highest weight wins.

   <li>Specificity. The <a href="/TR/css3-selectors">Selectors module</a> <a
    href="#SELECT" rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a> describes
    how to compute the specificity. The declaration with the highest
    specificity wins.

   <li>Order of appearance. The last declaration wins. Rules in imported
    style sheets are considered to be before any rules in the style sheet
    itself. Rules in two imported style sheets are considered to be in the
    same order as the @import rules themselves.
  </ol>

  <p>The sorting process continues until one winning declaration is found.

<
4B6C
div>  <h3 id=inheritance><span class=secno>3.2. </span> Inheritance</h3>

  <p>Inheritance is a way of propagating property values from parent elements
   to their children. Inheritance means that the specified value of a given
   element/property is copied from the parent element's computed value for
   the same property.

  <p>The root element, which has no parent element, inherits the initial
   value of the property. <!--
<p>The page context inherits from the initial value of the property. Margin boxes inherit from the page context.
-->


  <p>Pseudo-elements inherit according to a fictional tag sequence described
   for each pseudo-element <a href="#SELECT"
   rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>.

  <p>Some properties are said to be <dfn id=inherited
   title=inheritance>inherited</dfn>. This means that, unless a value is
   specified for the element/property combination, the value will be
   determined by inheritance.

  <p>All properties accept the &lsquo;<code class=css>inherit</code>&rsquo;
   value which explicitly specifies that the value will be determined by
   inheritance. The &lsquo;<code class=css>inherit</code>&rsquo; value can be
   used to strengthen inherited values in the cascade, and it can also be
   used on properties that are not normally inherited. If the &lsquo;<code
   class=css>inherit</code>&rsquo; value is set on the root element, the
   property is assigned its initial value.

  <h3 id=initial><span class=secno>3.3. </span> Initial value</h3>

  <p>Each property has an <dfn id=initial-value>initial value</dfn> which
   becomes the specified value when cascading and inheritance does not yield
   a value. Note, the initial value can be explicitly specified with the
   &lsquo;<code class=css>initial</code>&rsquo; keyword which all properties
   accept. [[CSS3VALUE]] <!--
David's proposal for ''default'' value:
  http://lists.w3.org/Archives/Member/w3c-css-wg/2002OctDec/0191.html
-->


  <h3 id=computing-weight><span class=secno>3.4. </span> Computing weight</h3>

  <p>In order to sort declarations in the cascading process, the weight of a
   declaration must be known. In CSS3, the weight of a declaration is based
   on the origin of the declaration and its marked level of importance.

  <p>CSS style sheets may have three different origins: author, user, and
   user agent (UA).

  <ul>
   <li><strong>Author:</strong> 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.

   <li><strong>User:</strong> 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 behave as if it did).

   <li><strong>User agent:</strong> <em>Conforming user agents</em> [add ref]
    must apply a "default style sheet" (or behave as if they did) prior to
    all other style sheets for a document. 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 "Appendix D. Default style sheet for HTML 4" <a href="#CSS21"
 
C054
   rel=biblioentry>[CSS21]<!--{{CSS21}}--></a> for a recommended default
    style sheet for HTML 4.0 documents.
  </ul>

  <p>Each CSS declaration can have two levels of importance: normal (which is
   default) and important (which must be marked). The CSS3 syntax module <a
   href="#CSS3SYN" rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a> describes
   how declarations are marked as important.

  <p>The weight of style sheets from the various origins, in ascending order,
   is:

  <ol>
   <li>User agent style sheets

   <li>User normal style sheets

   <li>Author normal style sheets

   <li>Author important style sheets

   <li>User important style sheets
  </ol>

  <p>By default, this strategy gives author declarations more weight than
   those of the user. It is therefore important that the UA give the user the
   ability to turn off the influence of a certain style sheet, e.g., through
   a pull-down menu.

  <p>Declarations in imported style sheets have lower weight than
   declarations in the style sheet from where they are imported. Imported
   style sheets can themselves import and override other style sheets,
   recursively, and the same precedence declarations apply.

  <p>If the user agent chooses to honor presentational hints from other
   sources than style sheets, these hints must be given the same weight as
   the user agent's default style sheet. This rule is intended for
   presentational hints in HTML.

  <p class=note>Note that non-CSS presentational hints had a higher weight in
   CSS2.

  <h2 id=conformance><span class=secno>4. </span>Conformance</h2>

  <p>[TBD]

  <h2 class=no-num id=acknowledgments>Acknowledgments</h2>

  <p>David Baron contributed to this specification.

  <h2 class=no-num id=references>References</h2>

  <h3 class=no-num id=normative-references>Normative references</h3>
  <!--begin-normative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS3SYN>[CSS3SYN]

   <dd>L. David Baron. <a
    href="http://www.w3.org/TR/2003/WD-css3-syntax-20030813"><cite>CSS3
    module: Syntax.</cite></a> 13 August 2003. W3C Working Draft. (Work in
    progress.) URL: <a
    href="http://www.w3.org/TR/2003/WD-css3-syntax-20030813">http://www.w3.org/TR/2003/WD-css3-syntax-20030813</a>
    </dd>
   <!---->

   <dt id=CSS3VAL>[CSS3VAL]

   <dd>H&#229;kon Wium Lie; Tab Atkins; Elika J. Etemad. <a
    href="http://www.w3.org/TR/2011/WD-css3-values-20110906/"><cite>CSS
    Values and Units Module Level 3.</cite></a> 6 September 2011. W3C Working
    Draft. (Work in progress.) URL: <a
    href="http://www.w3.org/TR/2011/WD-css3-values-20110906/">http://www.w3.org/TR/2011/WD-css3-values-20110906/</a>
    </dd>
   <!---->

   <dt id=MEDIAQ>[MEDIAQ]

   <dd>H&#229;kon Wium Lie; et al. <a
    href="http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/"><cite>Media
    Queries.</cite></a> 27 July 2010. W3C Candidate Recommendation. (Work in
    progress.) URL: <a
    href="http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/">http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/</a>
    </dd>
   <!---->

   <dt id=SELECT>[SELECT]

   <dd>Tantek &#199;elik; et al. <a
    href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/"><cite>Selectors
    Level 3.</cite></a> 29 September 2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/">http://www.w3.org/TR/2011/REC-css3-selectors-20110929/</a>
    </dd>
   <!---->
  </dl>
  <!--end-normative-->

  <h3 class=no-num id=other-references>Other references</h3>
  <!--begin-informative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS21>[CSS21]

   <dd>Bert Bos; et al. <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
    Sheets Level 2 Revision 1 (CSS&#160;2.1) Specification.</cite></a> 7 June
    2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
    </dd>
   <!---->

   <dt id=CSS3PAGE>[CSS3PAGE]

   <dd>H&#229;kon Wium Lie; Melinda Grant. <a
    href="http://www.w3.org/TR/2006/WD-css3-page-20061010"><cite>CSS3 Module:
    Paged Media.</cite></a> 10 October 2006. W3C Working Draft. (Work in
    progress.) URL: <a
    href="http://www.w3.org/TR/2006/WD-css3-page-20061010">http://www.w3.org/TR/2006/WD-css3-page-20061010</a>
    </dd>
   <!---->

   <dt id=HTML401>[HTML401]

   <dd>Dave Raggett; Arnaud Le Hors; Ian Jacobs. <a
    href="http://www.w3.org/TR/1999/REC-html401-19991224"><cite>HTML 4.01
    Specification.</cite></a> 24 December 1999. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/1999/REC-html401-19991224">http://www.w3.org/TR/1999/REC-html401-19991224</a>
    </dd>
   <!---->
  </dl>
  <!--end-informative-->

  <h2 class=no-num id=index>Index</h2>
  <!--begin-index-->

  <ul class=indexlist>
   <li>Alternate style sheets, <a href="#alternate-style-sheets"
    title="Alternate style sheets"><strong>2.2.</strong></a>

   <li>alternative style sheets, <a href="#alternative-style-sheets"
    title="alternative style sheets"><strong>2.2.</strong></a>

   <li>conditional import, <a href="#conditional-imports"
    title="conditional import"><strong>2.1.</strong></a>

   <li>&lsquo;<code class=css>@import</code>&rsquo;, <a href="#import"
    title="'@import'"><strong>2.1.</strong></a>

   <li>inheritance, <a href="#inherited"
    title=inheritance><strong>3.2.</strong></a>

   <li>initial value, <a href="#initial-value"
    title="initial value"><strong>3.3.</strong></a>

   <li>@media, <a href="#media" title="@media"><strong>2.3.</strong></a>

   <li>media-dependent import, <a href="#conditional-imports"
    title="media-dependent import"><strong>2.1.</strong></a>

   <li>persistent style sheets, <a href="#persistent-style-sheets"
    title="persistent style sheets"><strong>2.2.</strong></a>

   <li>preferred style sheet, <a href="#preferred-style-sheet"
    title="preferred style sheet"><strong>2.2.</strong></a>

   <li>winning declaration, <a href="#winning-declaration"
    title="winning declaration"><strong>3.1.</strong></a>
  </ul>
  <!--end-index-->

  <h2 class=no-num id=property-index>Property index</h2>
  <!--begin-properties-->

  <table class=proptable>
   <thead>
    <tr>
     <th>Property

     <th>Values

     <th>Initial

     <th>Applies&nbsp;to

     <th>Inh.

     <th>Percentages

     <th>Media

   <tbody>
  </table>
  <!--end-properties-->