csswg-drafts/css3-conditional/Overview.src.html at 447bd1ee39e4a794ababe51eb4223bdcfaee8be8 · 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 profile="http://www.w3.org/2006/03/hcard">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Conditional Rules Module Level 3</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
  <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
<!--
  <script src="http://test.csswg.org/harness/annotate.js#CSS3-CONDITIONAL_DEV"
        type="text/javascript" defer></script
-->
</head>
<body class="h-entry">

<div class="head">
<!--logo-->

<h1 class="p-name">CSS Conditional Rules Module Level 3</h1>

<h2 class="no-num no-toc">[LONGSTATUS] <span class="dt-updated"><span class="value-title" title="[CDATE]">[DATE]</span></span></h2>
<dl>
  <dt>This version:
    <dd><a class="u-url" href="[VERSION]">
    http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>

  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>

  <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>

  <dt>Previous version:
    <dd><a rel="previous" href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">http://www.w3.org/TR/2012/WD-css3-conditional-20120911/</a></dd>

  <dt>Editors:
    <dd class="p-author h-card vcard"><a class="p-name fn u-url url" rel="author" href="http://dbaron.org/">L. David Baron</a>,
      <a class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>

  <dt>Issues list:
    <dd>Maintained in document (only editor's draft is current)

    <dt>Feedback:

    <dd><a
     href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
     with subject line “<kbd>&#x5b;[SHORTNAME]&#x5d; <var>… message topic
     …</var></kbd>”

  <dt>Test suite:
    <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>; no built test suite yet

</dl>

<!--copyright-->

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

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

  <p>CSS is a language for describing the rendering of structured documents
  (such as HTML and XML) on screen, on paper,  in speech, etc.
  <span class="p-summary">
  This module
  contains the features of CSS for conditional processing of parts of
  style sheets, conditioned on capabilities of the processor or the
  document the style sheet is being applied to.
  It includes and extends the functionality of CSS level&nbsp;2 [[!CSS21]],
  which builds on CSS level&nbsp;1 [[CSS1]].
  The main extensions compared to level&nbsp;2 are
  allowing nesting of certain at-rules inside '@media',
  and the addition of the '@supports'
  rule for conditional processing.
  </span>

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

<!--status-->

<p>The following features are at risk:
<ul>
  <li>The inclusion of '@font-face' rules and
  '@keyframes' rules as allowed within all of the @-rules in
  this specification is at risk, though only because of the relative
  rates of advancement of specifications.  If this specification is able
  to advance faster than one or both of the specifications defining
  those rules, then the inclusion of those rules will move from this
  specification to the specification defining those rules.</li>

  <li>The addition of support for @-rules inside of conditional grouping
  rules is at risk; if interoperable implementations are not found, it
  may be removed to advance the other features in this specification to
  Proposed Recommendation.</li>

  <li>The '@supports' rule is at risk; if interoperable
  implementations are not found, it may be removed to advance the other
  features in this specification to Proposed Recommendation.</li>
</ul>

<p>This is a <strong>Last Call Working Draft</strong>.
The deadline for comments is <strong>10 January 2013</strong>.

<!--

  Things to go in level 4:

  * Create some way to put these new conditional things on an @import.
  * The @document rule (commented out, down below).

-->

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

<!--toc-->

<h2 id="introduction">Introduction</h2>

<h3 id="context">Background</h3>

  <p><em>This section is not normative.</em>

  <p>[[!CSS21]] defines one type of conditional group rule, the
  '@media' rule, and allows only rulesets (not other @-rules)
  inside of it.  The '@media' rule provides the ability to
  have media-specific style sheets, which is also provided by style
  sheet linking features such as '@import' and
  <code class="html">&lt;link&gt;</code>.  The restrictions on the contents of
  '@media' rules made them less useful; they have forced authors
  using CSS features involving @-rules in media-specific style sheets to
  use separate style sheets for each medium.</p>

  <p>This specification extends the rules for the contents of
  conditional group rules to allow other @-rules, which enables authors
  to combine CSS features involving @-rules with media specific style
  sheets within a single style sheet.</p>

  <p>This specification also defines an additional type of conditional
  group rule, '@supports', to
  address author and user requirements.</p>

  <p>The '@supports' rule allows CSS to be conditioned on
  implementation support for CSS properties and values.  This rule makes
  it much easier for authors to use new CSS features and provide good
  fallback for implementations that do not support those features.  This
  is particularly important for CSS features that provide new layout
  mechanisms, and for other cases where a set of related styles needs to
  be conditioned on property support.</p>

<h3 id="placement">Module Interactions</h3>

  <p>This module replaces and extends the '@media' rule
  feature defined in [[!CSS21]] section <var>7.2.1</var> and
  incorporates the modifications previously made non-normatively by
  [[!MEDIAQ]] section <var>1</var>.</p>

  <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
  and [[!CSS3-ANIMATIONS]], but that dependency is only on the
  assumption that those modules will advance ahead of this one.  If this
  module advances faster, then the dependency will be reversed.</p>

<h3 id="conventions">Document Conventions</h3>

  <p>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.

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

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

  <div class="example">
    <p>This is an example of an informative example.</p>
  </div>

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

  <p class="note">Note, this is an informative note.</p>

<h2 id="processing">Processing of conditional group rules</h2>

<p>This specification defines some CSS @-rules, called <dfn>conditional
group rules</dfn>, that associate a condition with a group of other
CSS rules.  These different rules allow testing different types of
conditions, but share common behavior for how their contents are used
when the condition is true and when the condition is false.</p>

<div class="example">
<p>For example, this rule:</p>
<pre>@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}</pre>
<p>causes a particular CSS rule (making elements with ID "navigation" be
display:none) apply only when the style sheet is used for a print
medium.
</div>

<p>Each conditional group rule has a condition, which at any time
evaluates to true or false.  When the condition is true, CSS processors
<strong>must</strong> apply the rules inside the group rule as though
they were at the group rule's location; when the condition is false, CSS
processors <strong>must not</strong> apply any of rules inside the group
rule.  The current state of the condition does not affect the CSS object
model, in which the contents of the group rule always remain within the
group rule.</p>

<p>This means that when multiple conditional group rules are nested,
a rule inside of both of them applies only when all of the rules'
conditions are true.</p>

<div class="example">For example, with this set of nested rules:
<pre>@media print { // rule (1)
  /* hide navigation controls when printing */
  #navigation { display: none }
  @media (max-width: 12cm) { // rule (2)
    /* keep notes in flow when printing to narrow pages */
    .note { float: none }
  }
}</pre>
the condition of the rule marked (1) is true for print media, and the
condition of the rule marked (2) is true when the width of the display
area (which for print media
6C3A
 is the page box) is less than or equal to
12cm.  Thus the rule ''#navigation { display: none }'' applies
whenever this style sheet is applied to print media, and the rule
''.note { float: none }'' is applied only when the style sheet
is applied to print media <em>and</em> the width of the page box is less
than or equal to 12 centimeters.</div>

<p>When the condition for a conditional group rule changes, CSS
processors <strong>must</strong> reflect that the rules now apply or no
longer apply, except for properties whose definitions define effects of
computed values that persist past the lifetime of that value (such as
for some properties in [[CSS3-TRANSITIONS]] and
[[!CSS3-ANIMATIONS]]).</p>

<h2 id="contents-of">Contents of conditional group rules</h2>

<p>The syntax of each conditional group rule consists of some syntax
specific to the type of rule followed by a <dfn>group rule body</dfn>,
which is a block (pair of braces) containing a sequence of rules.</p>

<p>A group rule body is allowed to contain rulesets and any @-rules that
are allowed at the top level of a style sheet before and after a
ruleset.  This means that @-rules that must occur at the beginning of
the style sheet (such as '@charset', '@import',
and '@namespace' rules) are not allowed inside of conditional group
rules.  Conditional group rules can be nested.</p>

<p>In terms of the grammar, this specification defines the following
productions for use in the grammar of conditional group rules:</p>

<pre><dfn>nested_statement</dfn>
  : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
    <i>supports_rule</i>
  ;

<dfn>group_rule_body</dfn>
  : '{' S* <i>nested_statement</i>* '}' S*
  ;</pre>
<p>
in which all the productions are defined in that grammar with the
exception of <code>font_face_rule</code>
defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
[[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
defined in this specification.</p>

<p>In general, future CSS specifications that add new @-rules that are
not forbidden to occur after some other types of rules should modify
this <code>nested_statement</code> production to keep the grammar
accurate.</p>

<p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
conditional group rules.</p>

<p>CSS processors <strong>must</strong> ignore rules that are not
allowed within a group rule, and <strong>must</strong> handle invalid
rules inside of group rules as described in <a
href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
4.2 (Rules for handling parsing errors)</a>, <a
href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
(At-rules)</a>, and <a
href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
(Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>

<h2 id="use">Placement of conditional group rules</h2>

<p>Conditional group rules are allowed at the top-level of a style
sheet, and inside other conditional group rules.  CSS processors
<strong>must</strong> process such rules as <a
href="#processing">described above</a>.</p>

<p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
''@import'', or ''@namespace'' rules) are also not allowed after a
conditional group rule.  Therefore, style sheets <strong>must
not</strong> place such rules after a conditional group rules, and CSS
processors <strong>must</strong> ignore such rules.</p>

<h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>

<p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
condition is a media query.  It consists of the at-keyword
'@media' followed by a (possibly empty) media query list (as
defined in [[!MEDIAQ]]), followed by a group rule body.  The condition
of the rule is the result of the media query.</p>

<div class="example">
<p>This '@media' rule:</p>
<pre>@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}</pre>
<p>has the condition
''screen and (min-width: 35em), print and (min-width: 40em)'',
which is true for screen displays
whose viewport is at least 35 times the initial font size
and for print displays
whose viewport is at least 40 times the initial font size.
When either of these is true,
the condition of the rule is true,
and the rule
''#section_navigation { float: left; width: 10em; }''
is applied.</p>
</div>

<p>In terms of the grammar, this specification extends the
<code>media</code> production in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) into:
<pre><dfn>media</dfn>
  : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
  ;</pre>
<p>where the <code>group_rule_body</code> production is defined in this
specification, the <code>media_query_list</code> production is defined
in [[!MEDIAQ]], and the others are defined in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G).

<h2 id="at-supports">Feature queries: the '@supports' rule</h2>

<p>The <dfn>'@supports' rule</dfn> is a conditional group
rule whose condition tests whether the user agent supports CSS
property:value pairs.  Authors can use it to write style sheets that use
new features when available but degrade gracefully when those features
are not supported.  CSS has existing mechanisms for graceful
degradation, such as ignoring unsupported properties or values, but
these are not always sufficient when large groups of styles need to be
tied to the support for certain features, as is the case for use of new
layout system features.</p>

<p>The syntax of the condition in the '@supports' rule is
slightly more complicated than for the other conditional group rules
(though has some similarities to media queries) since:</p>
<ul>
  <li>negation is needed so that the new-feature styles and the fallback
  styles can be separated (within the forward-compatible grammar's rules
  for the syntax of @-rules), and not required to override each other</li>
  <li>conjunction (and) is needed so that multiple required features can
  be tested</li>
  <li>disjunction (or) is needed when there are multiple alternative
  features for a set of styles, particularly when some of those
  alternatives are vendor-prefixed properties or values</li>
</ul>

<p>Therefore, the syntax of the '@supports' rule allows
testing for property:value pairs, and arbitrary conjunctions (and),
disjunctions (or), and negations (not) of them.</p>

<p>This extends the lexical scanner in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>
@{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
{O}{R}                    {return <dfn>OR</dfn>;}
</pre>

<p>This then extends the grammar in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
using the lexical scanner there, with the additions of
<code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
<code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
tokens defined in the Media Queries specification [[!MEDIAQ]]
and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
and with
<code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
<code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
productions
and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
taken from the core syntax of CSS defined in
<a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
by adding:</p>

<pre><dfn>supports_rule</dfn>
  : <i>SUPPORTS_SYM</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>supports_condition</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>group_rule_body</i>
  ;

<dfn>supports_condition</dfn>
  : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
    <i>supports_condition_in_parens</i>
  ;

<dfn>supports_condition_in_parens</dfn>
  : ( '(' <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>supports_condition</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* ')' ) | <i>supports_declaration_condition</i> |
    <i>general_enclosed</i>
  ;

<dfn>supports_negation</dfn>
  : <a href="http://www.w3.org/TR/css3-mediaqueries/#syntax"><i>NOT</i></a> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i>
  ;

<dfn>supports_conjunction</dfn>
  : <i>supports_condition_in_parens</i> ( <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <a href="http://www.w3.org/TR/css3-mediaqueries/#syntax"><i>AND</i></a> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i> )+
  ;

<dfn>supports_disjunction</dfn>
  : <i>supports_condition_in_parens</i> ( <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>OR</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i> )+
  ;

<dfn>supports_declaration_condition</dfn>
  : '(' <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>declaration</i></a> ')'
  ;

<dfn>general_enclosed</dfn>
  : ( <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>FUNCTION</i></a> | '(' ) ( <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>any</i></a> | <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>unused</i></a> )* ')'
  ;
</pre>

<p>
  Implementations <strong>must</strong> parse ''@supports'' rules
  based on the above grammar,
  and when interpreting the above grammar,
  <strong>must</strong> match the production before an <code>|</code> operator
  in preference to the one after it.
</p>

<p>
  The above grammar is purposely very loose for forwards-compatibility reasons,
  since the <code>general_enclosed</code> production
  allows for substantial future extensibility.
  Any ''@supports'' rule that does not parse according to the grammar above
  (that is, a rule that does not match this loose grammar
  which includes the <code>general_enclosed</code> production)
  is invalid.
  Style sheets <strong>must not</strong> use such a rule and
  processors <strong>must</strong> ignore such a rule (including all of its contents).

<p>Each of these grammar terms is associated with a boolean result, as
follows:</p>
<dl>
<dt>supports_condition</dt>
<dd>
  The result is the result of the single child term.
</dd>

<dt>supports_condition_in_parens</dt>
<dd>
  The result is the result of the single <code>supports_condition</code>
  or <code>supports_declaration_condition</code> child term.
</dd>

<dt>supports_negation</dt>
<dd>
  The result is the <em>negation</em> of the result of the
  <code>supports_condition_in_parens</code> child term.
</dd>

<dt>supports_conjunction</dt>
<dd>
  T
10A9C
he result is true if the result of <em>all</em> of the
  <code>supports_condition_in_parens</code> child terms is true;
  otherwise it is false.
</dd>

<dt>supports_disjunction</dt>
<dd>
  The result is true if the result of <em>any</em> of the
  <code>supports_condition_in_parens</code> child terms is true;
  otherwise it is false.
</dd>

<dt>supports_declaration_condition</dt>
<dd>
  The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
  within the parentheses.
</dd>

<dt>general_enclosed</dt>
<dd>
  The result is always false.
  Additionally, style sheets <strong>must not</strong>
  write ''@supports'' rules
  that match this grammar production.
  (In other words, this production exists only for future extensibility,
  and is not part of the description of a valid style sheet
  in this level of the specification.)
  <span class="note">Note that future levels may define functions
  or other parenthesized expressions that can evaluate to true.</span>
</dd>
</dl>

<p>The condition of the '@supports' rule is the result of the
<code>supports_condition</code> term that is a child of the
<code>supports_rule</code> term.</p>

<div class="example">
<p>For example, the following rule</p>
<pre>@supports ( display: flexbox ) {
  body, #navigation, #content { display: flexbox; }
  #navigation { background: blue; color: white; }
  #article { background: white; color: black; }
}</pre>
<p>applies the rules inside the '@supports' rule only when
''display: flexbox'' is supported.</p>
</div>

<div class="example">
<p>The following example shows an additional '@supports' rule that can
be used to provide an alternative for when ''display: flexbox'' is not
supported:</p>
<pre>@supports not ( display: flexbox ) {
  body { width: 100%; height: 100%; background: white; color: black; }
  #navigation { width: 25%; }
  #article { width: 75%; }
}</pre>
<p>Note that the 'width' declarations may be harmful to the
flexbox-based layout, so it is important that they be present only in
the non-flexbox styles.</p>
</div>

<div class="example">
<p>The following example checks for support for the 'box-shadow'
property, including checking for support for vendor-prefixed versions of
it.  When the support is present, it specifies both 'box-shadow' (with
the prefixed versions) and 'color' in a way what would cause the text to
become invisible were 'box-shadow' not supported.</p>
<pre>@supports ( box-shadow: 2px 2px 2px black ) or
          ( -moz-box-shadow: 2px 2px 2px black ) or
          ( -webkit-box-shadow: 2px 2px 2px black ) or
          ( -o-box-shadow: 2px 2px 2px black ) {
  .outline {
    color: white;
    -moz-box-shadow: 2px 2px 2px black;
    -webkit-box-shadow: 2px 2px 2px black;
    -o-box-shadow: 2px 2px 2px black;
    box-shadow: 2px 2px 2px black; /* unprefixed last */
  }
}</pre></div>

<p>To avoid confusion between ''and'' and ''or'', the syntax requires
that both ''and'' and ''or'' be specified explicitly (rather than, say,
using commas or spaces for one of them).  Likewise, to avoid confusion
caused by precedence rules, the syntax does not allow ''and'', ''or'',
and ''not'' operators to be mixed without a layer of parentheses.</p>

<div class="example">
<p>For example, the following rule is not valid:
<pre class="illegal">@supports (transition-property: color) or
          (animation-name: foo) and
          (transform: rotate(10deg)) {
  // ...
}</pre>
<p>Instead, authors must write one of the following:</p>
<pre>@supports ((transition-property: color) or
           (animation-name: foo)) and
          (transform: rotate(10deg)) {
  // ...
}</pre>
<pre>@supports (transition-property: color) or
          ((animation-name: foo) and
           (transform: rotate(10deg))) {
  // ...
}</pre>
</div>

<p>Furthermore, whitespace is required after a ''not'' and on both
sides of an ''and'' or ''or''.</p>

<p>The declaration being tested must always occur within parentheses,
when it is the only thing in the expression.<p>

<div class="example">
<p>For example, the following rule is not valid:
<pre class="illegal">@supports display: flexbox {
  // ...
}</pre>
<p>Instead, authors must write:</p>
<pre>@supports (display: flexbox) {
  // ...
}</pre>
</div>

<p>The syntax allows extra parentheses when they are not needed.  This
flexibility is sometimes useful for authors (for example, when
commenting out parts of an expression) and may also be useful for
authoring tools.</p>

<div class="example">
<p>For example, authors may write:</p>
<pre>@supports ((display: flexbox)) {
  // ...
}</pre>
</div>

<p>A trailing ''!important'' on a declaration being tested is allowed,
though it won't change the validity of the declaration.

<div class="example">
<p>For example, the following rule is valid:
<pre>@supports (display: flexbox !important) {
  // ...
}</pre>
</div>

<h3 id="support-definition">Definition of support</h3>

<p>For forward-compatibility,
<a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
(Declarations and properties)</a> of [[!CSS21]]
defines rules for handling invalid properties and values.
CSS processors that
do not implement or partially implement a specification
<strong>must</strong> treat any part of a value that they
do not implement, or
do not have a usable level of support for,
as invalid according to this rule
for handling invalid properties and values,
and therefore <strong>must</strong> discard the declaration as a parse error.</p>

<p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
a declaration (consisting of a property and value) if it accepts that
declaration (rather than discarding it as a parse error).
If a processor does not implement, with a usable level of support,
the value given,
then it <strong>must not</strong>
accept the declaration or claim support for it.</p>

<p class="note">Note that properties or values
whose support is effectively disabled by user preferences
are still considered as supported by this definition.
For example, if a user has enabled a high-contrast mode
that causes colors to be overridden,
the CSS processor is still considered to support the 'color' property
even though declarations of the 'color' property may have no effect.
On the other hand, a developer-facing preference
whose purpose is to enable or disable support for an experimental CSS feature
does affect this definition of support.</p>

<p>These rules (and the equivalence between them) allow
authors to use fallback (either in the [[CSS1]] sense of declarations
that are overridden by later declarations or with the new capabilities
provided by the ''@supports'' rule in this specification) that works
correctly for the features implemented.  This applies especially to
compound values; implementations must implement all parts of the value
in order to consider the declaration supported, either inside a ruleset
or in the declaration condition of an ''@supports'' rule.</p>

<!--
<h2 id="at-document">Document queries: the '@document' rule</h2>

<p>The <dfn>'@document' rule</dfn> is a conditional group
rule whose condition depends on the
<a href="#url-of-doc">URL of the document being styled</a>.
This allows style sheets, particularly user style sheets, to have styles
that only apply to a set of pages rather than to all pages using the
style sheet.</p>

<p class="issue">Given that this @-rule is intended primarily for user
style sheets, what should this specification say about its use in author
style sheets?  Should it be forbidden?  Should use instead be
discouraged?  Or should this specification remain neutral on the
topic, since there are valid uses in author style sheets?</p>

<p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
the URI at which the document is located, excluding any fragment
identifiers.  (This means, for example, that HTTP redirects have been
followed.)  If the styles are being applied inside a complete document
embedded into the presentation of another (e.g., [[HTML5]]&#39;s <code
class="html">iframe</code>, <code class="html">object</code>, or <code
class="html">img</code> elements), the relevant URI is that of the
frame, not of its container.  However, if content from other documents
is mixed in via mechanisms that mix content from one document into
another (e.g., [[SVG11]]&#39;s <code>use</code> element), then the
address of the container document is used.</p>

<p class="note">Note:  In [[HTML5]], this is the
<a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
of a document in a
<a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>

<div class="issue">What form of normalization is done on URLs and domains
before matching?  In particular, this specification needs to describe:
<ul>
<li>what form is used for the <a href="#url-of-doc">URL of the document
being styled</a> (and what has been normalized in that form)</li>
<li>what normalization (if any) happens to the argument of each of the match
functions before the comparison that they describe and</li>
<li>whether the
comparison algorithm used is string comparison or some other URL
comparison algorithm.</li></ul></div>

<p>The '@document' rule's condition is written as a
comma-separated list of <dfn>URL matching functions</dfn>, and the
condition evaluates to true whenever any one of those functions
evaluates to true.  The following URL matching functions are
permitted:</p>

<dl>
  <dt><dfn id="url-exact" title="url()|URL matching functions::exact">&lt;url&gt;</dfn></dt>

  <dd>
    <p>The 'url()' function is the <dfn>exact url matching
    function</dfn>.  It evaluates to true whenever the <a
    href="#url-of-doc">URL of the document being styled</a> is exactly
    the URL given.</p>

    <p class="Note">The 'url()' function, since it is a core syntax
    element in CSS, is allowed (subject to different character
    limitations and thus escaping requirements) to contain an unquoted
    value (in addition to the string values that are allowed as
    arguments for all four functions).</p>

    <div class="example">
      <p>For example, this rule:</p>
<pre>@document url("http://www.w3.org/Style/CSS/") {
  #summary { background: yellow; color: black}
}</pre>
      <p>styles the <code class="html">summary</code> element on the page
      <code>http://www.w3.org/Style/CSS/</code>, but not on any other
      pages.</p>
    </div>
  </dd>

  <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'url-prefix()' function is the <dfn>url prefix
    matching function</dfn>.  It evaluates to true whenever the
    <a href="#url-of-doc">URL of the document being styled</a>
    has the argument to the function as an
    initial substring (which is true when the two strings are equal).
    When the argument is the empty string, it evaluates to true for all
    documents.</p>
    <div class="example">
      <p>For example, this rule:</p>
<pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
  #summary { background: yellow; color: black}
}</pre>
      <p>styles the <code class="html">summary</code> element on the page
      <code>http://www.w3.org/Style/CSS/</code> and on the page
      <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
      affect the page <code>http://www.w3.org/</code> or the page
      <code>http://www.example.com/Style/CSS/</code>.</p>
    </div>
  </dd>

  <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'domain()' function is the <dfn>domain
    matching function</dfn>.  It evaluates to true whenever
    the <a href="#url-of-doc">URL of the document being styled</a>
    has a host subcomponent (as defined in [[!URI]])
    and that host subcomponent is exactly the argument to the
    'domain()' function or a final substring of the host
    component is a period (U+002E) immediately followed by the argument
    to the 'domain()' function.</p>
    <div class="example">
      <p>For example, this rule:</p>
<pre>@document domain("w3.org") {
  body { font-size: 16px ! important }
}</pre>
      <p>changes the font size of the body element for pages such as
      <code>http://www.w3.org/Style/CSS/</code> and
      <code>http://w3.org/Style/CSS/</code> and
      <code>http://lists.w3.org/Archives/Public/www-style/</code>
      but it does not affect the page
      <code>http://www.example.com/Style/CSS/</code>.</p>
    </div>
  </dd>

  <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The contents of the &lt;string&gt; argument <strong>must</strong>
    match the JavaScript <code>Pattern</code> production
    ([[!ECMA-262-5.1]], section 15.10.1).  However,
    failing to do so is not a CSS syntax error and does not trigger any
    error handling for CSS syntax errors.</p>

    <p>The ''regexp()'' function evaluates to true whenever the string
    argument compiled as a JavaScript regular expression with the
    <code>global</code>, <code>ignoreCase</code> and
    <code>multiline</code> flags <em>disabled</em>
    (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
    compiles successfully and the resulting regular expression matches
    the entirety of the
    <a href="#url-of-doc">URL of the document being styled</a>.</p>

    <p class="note">Note that regular expression must match the entire
    URL, not just a part of it.</p>

    <p class="note">Note that this definition intentionally matches the
    behavior of the <a
    href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
    attribute</a> on the <code class="html">input</code> element
    in [[HTML5]].</p>

    <div class="example">
      <p>For example, this rule:</p>
<pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
  body { font-size: 20px ! important }
}</pre>
      <p>changes the font size of the body element for pages such as
      <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
      <p class="note">Note that the backslashes in the regular
      expression require CSS escaping as ''\\''.</p>
    </div>
  </dd>

</dl>

<p>Implementations <strong>must</strong> treat any unknown URL matching
functions as a syntax error, and thus ignore the '@document' rule.
<span class="issue">Should we instead have more complicated error
handling rules to make forward-compatibility work differently, or is
this rule the best solution for such future expansion anyway?</span></p>

<div class="issue">This syntax doesn't offer any ability to do negations,
which has been requested in <a
href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
349813</a>.  Use cases that people have wanted negations for
include:
<ul>
  <li>User style sheets that want a particular rule in general, but know
  that that rule does more harm than good on specific sites.</li>
  <li>Authors who have a rule that they want to apply to most of their
  pages, but wish to make a few exceptions for.</li>
</ul>
</div>

<p>This extends the lexical scanner in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>@{D}{O}{C}{U}{M}{E}{N}{T}	{return DOCUMENT_SYM;}</pre>
<p>and the grammar by adding</p>
<pre><dfn>document_rule</dfn>
  : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
  ;

<dfn>url_match_fn</dfn>
  : (URI | FUNCTION S* STRING S* ')' ) S*
  ;</pre>
-->


<h2 id="apis">APIs</h2>

<h3 id='extentions-to-cssrule-interface'>
Extensions to the <code>CSSRule</code> interface</h3>

<p>The <code>CSSRule</code> interface is extended as follows:

<pre class='idl'>partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;
    <!--
    const unsigned short DOCUMENT_RULE = 13;
    -->
}</pre>


<h3 id='the-cssgroupingrule-interface'>
The <code>CSSGroupingRule</code> interface</h3>

<p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.

<pre class='idl'>interface CSSGroupingRule : CSSRule {
    readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
    unsigned long insertRule (DOMString rule, unsigned long index);
    void deleteRule (unsigned long index);
}</pre>

<dl class='idl-attributes'>
  <dt><code>cssRules</code> of type <code><a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a></code>, readonly
  <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
    object for the list of CSS rules nested inside the grouping rule.
</dl>

<dl class='idl-methods'>
  <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
    <code>unsigned long</code>
  <dd>
    The <code>insertRule</code> operation must
    insert a CSS rule <var>rule</var>
    into the CSS rule list returned by <code>cssRules</code>,
    such that the inserted rule will be at position <var>index</var>,
    and any rules previously at <var>index</var> or higher
    will increase their index by one.
    It must throw INDEX_SIZE_ERR
    if index is greater than <code>cssRules.length</code>.
    It must throw SYNTAX_ERR
    if the rule has a syntax error and is unparseable;
    this does not include syntax errors handled by error handling rules
    for constructs inside of the rule,
    but this does include cases where the string given
    does not parse into a single CSS rule (such as when the string is empty)
    or where there is anything other than whitespace or comments
    after that single CSS rule.
    It must throw HIERARCHY_REQUEST_ERR
    if the rule cannot be inserted at the location specified,
    for example, if an ''@import'' rule is inserted inside a group rule.

    <p>The return value is the <var>index</var> parameter.

  <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
  <dd>
    The <code>deleteRule</code> operation must
    remove a CSS rule from
    the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
    It must throw INDEX_SIZE_ERR
    if index is greater than or equal to <code>cssRules.length</code>.
</dl>


<h3 id="the-cssconditionrule-interface">
The <code>CSSConditionRule</code> interface</h3>

<p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
  which consist of a condition and a statement block.

<pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
    attribute DOMString conditionText;
}</pre>

<dl class='idl-attributes'>

  <dt><code>conditionText</code> of type <code>DOMString</code>
  <dd>
    <p>The <code>conditionText</code> attribute represents
    the condition of the rule.
    Since what this condition does
    varies between the derived interfaces of <code>CSSConditionRule</code>,
    those derived interfaces
    may specify different behavior for this attribute
    (see, for example, <code>CSSMediaRule</code> below).
    In the absence of such rule-specific behavior,
    the following rules apply:</p>

    <p>The <code>conditionText</code> attribute, on getting, must return
    the result of serializing the associated condition.

    <p>On setting the <code>conditionText</code> attribute these steps
      must be run:

    <ol>
      <li>Trim the given value of white space.
      <li>If the given value matches the grammar of the
        appropriate condition production for the given rule,
        replace the associated CSS condition with the given value.
      <li>Otherwise, do nothing.
    </ol>
</dl>


<h3 id="the-cssmediarule-interface">
The <code>CSSMediaRule</code> interface</h3>

<p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:

<pre class='idl'>interface CSSMediaRule : CSSConditionRule {
    readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
}</pre>

<dl class='idl-attributes'>
  <dt><code>media</code> of type <code><a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a&g
4126
t;</code>, readonly
  <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
    for the list of media queries specified with the ''@media'' rule.

  <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
  <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
    on getting, must return the value of <code>media.mediaText</code> on the rule.

    <p>Setting the <code>conditionText</code> attribute
      must set the <code>media.mediaText</code> attribute on the rule.
</dl>


<h3 id="the-csssupportsrule-interface">
The <code>CSSSupportsRule</code> interface</h3>

<p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>