csswg-drafts/css3-cascade/Overview.src.html at bee57939172a62a9170a2e55ff61cbb53ce65059 · 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 http-equiv="Content-Type" content="text/html">
<title>CSS3 module: Cascading and inheritance</title>

<link href="../default.css" rel="stylesheet" type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].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>
</head>

<body>

<div class="head">

<!--logo-->

<h1>CSS3 module: Cascading and inheritance</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>

<dl>
  <dt>This version:
    <dd><a
      href="[VERSION]">[VERSION]</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>[Here will be included the file "../copyright.inc"]</p>
<!--end-copyright-->

<hr title="Separator for header">
</div>

<h2 class="no-num no-toc">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
[[!CSS3SYN]] to parse the style sheets, on [[!SELECT]] to calculate
specificity, and on [[!MEDIAQ]] 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 [[!CSS3VAL]].


<h2 class="no-num no-toc">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 ''initial'' value
and the optional title for '@import' and '@media', 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">Table of contents</h2>
<!--toc-->

<hr>


<h2>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 <em>inheritance</em> or by looking at the property's
<em>initial value</em>.

<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 [[CSS3PAGE]].


<h2>Input to and output from this module</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 [[!MEDIAQ]]
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 [[!SELECT]])
<li>If the cascading process (described below) yields a winning
declaration and the value of the winning declaration is not ''initial''
or ''inherit'', the value of the winning declaration becomes the
specified value.

<li>Otherwise, if the value of the winning declaration is ''inherit'',
the inherited value (see below) becomes the specified value.

<li>Otherwise, if the value of the winning declaration is ''initial'',
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> [[!CSS3VAL]] module.


<h2 id="at-import">Importing style sheets</h2>

<p>The <dfn>'@import'</dfn> rule allows users to
import style rules from other style sheets.  Any '@import' rules must
follow all '@charset' rules and precede all other at-rules and 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(&hellip;) around it.

<div class="example">
<p>The following lines are equivalent in meaning and illustrate both
'@import' syntaxes (one with 'url()' 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 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 '@import' 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 [[!MEDIAQ]].

<p>In the absence of any media queries, the import is
unconditional. Specifying 'all' for the medium has the same effect.

<p>At the end of the '@import' 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>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
    '@import's without a name are imported, as well as the first
    '@import' that has a name and all other '@import's that have the
    same name.

  <li>
    <p>If the user has chosen a style be name, then all '@import's
    without a name are imported, as well as all '@import'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 '@import'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 '@import's (all shown in
<code><b>bold</b></code>) are used, the others are skipped:

<pre>
<b>@import "common1.css";</b>
@import &
8000
quot;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 '@import'
(if any) matches the media for which the UA renders the document.

<p>If an '@import' is skipped, then all '@import'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 '@media' and '@import' rules that occur in linked
  style sheets that don't have a name on the link, and

  <li>the names of '@media' and '@import' rules that occur in style
  sheets linked from '@import' 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 '@import'
rules.


<p class=note>HTML [[HTML401]] uses the term <dfn>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 '@import' or '@media'.
<dfn>Alternate style sheets</dfn> are all other
named style sheets. Finally, HTML calls style sheets without a name
<dfn>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., '@import url(foo.css) as
"Boxed"'.

<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?]</p>


<h2 id="at-media-rule">Named <code>@media</code> rules</h2>

<p>An <dfn>@media</dfn> rule specifies the target media types
(separated by commas) of a set of rules (delimited by curly braces).
The '@media' construct allows style sheet rules for various media in
the same style sheet:</p>

<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 '@media' is defined by the
Media Queries specification [[!MEDIAQ]].

<p>After the media queries, before the "{", there may be a string that
assigns a name to the '@media' 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 '@media' rules with a name, together with any '@import' rules
with the same name, present alternative style sheets. The UA must read
the rules inside some '@media' rules and skip others, as follows:

<ul>
  <li>
    <p>If the user has not chosen a style by name, the UA must read
    all '@media' 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 '@import' or the first named '@media', whichever comes
    first.

  <li>
    <p>If the user has chosen a style by name, the UA must read all
    '@media' 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&am
FE74
p;rdquo; style, the unnamed '@media' rule and all rules that
    are not in any '@media' (all shown in <code><b>bold</b></code>).
    We assume the UA uses the ''screen'' 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; '@media' rule, the unnamed '@media'
    rule and all rules that are not in any '@media' (all shown in
    <code><b>bold</b></code>). We assume the UA uses the ''screen''
    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>Cascading</h2>

<p>The purpose of cascading is to find one <dfn>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>
[[!SELECT]] 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.


<h2>Inheritance</h2>

<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 [[!SELECT]].


<p>Some properties are said to be <dfn
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 ''inherit'' value which explicitly
specifies that the value will be determined by inheritance. The
''inherit'' 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 ''inherit'' value is set on the root element, the
property is assigned its initial value.


<h2>Initial value</h2>

<p>Each property has an <dfn>initial value</dfn> which becomes the
specified value when cascading and inheritance does not yield a value.
Also, the initial value can be explicitly specified with the ''initial''
keyword which all properties accept.


<h2 id="computing-weight">Computing weight</h2>

<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" [[CSS21]] 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 [[!CSS3SYN]] 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>Conformance</h2>

<p>[TBD]

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

<p>David Baron contributed to thie specification.


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

<h3 class="no-num" id="normative-references">Normative references</h3>
<!--begin-normative-->


<p>[Here will be inserted the file "normative.inc"]</p>
<!--end-normative-->


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


<p>[Here will be inserted the file "informative.inc"]</p>
<!--end-informative-->


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

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


</body>
</html>