csswg-drafts/css-conditional/Overview.html at 8ff9e16e9e737649b0135548dabcd96be671dc95 · 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://dublincore.org/documents/2008/08/04/dc-html/ http://www.w3.org/2006/03/hcard">
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Conditional Rules Module Level 3</title>

  <link href="http://purl.org/dc/terms/" rel=schema.dcterms>
  <link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
   rel=dcterms.rights>
  <meta content="CSS Conditional Rules Module Level 3" name=dcterms.title>
  <meta content=text name=dcterms.type>
  <meta content=2013-06-08 name=dcterms.date>
  <meta content="L. David Baron" name=dcterms.creator>
  <meta content=W3C name=dcterms.publisher>
  <meta content="http://dev.w3.org/csswg/css3-conditional/"
   name=dcterms.identifier>
  <link href="../default.css" rel=stylesheet type="text/css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
   type="text/css">
  <script defer=defer
   src="http://test.csswg.org/harness/annotate.js#css-conditional-3_dev"
   type="text/javascript"></script>

 <body class=h-entry>
  <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 class=p-name>CSS Conditional Rules Module Level 3</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft <span
    class=dt-updated><span class=value-title title=20130608> 8 June
    2013</span></span></h2>

   <dl>
    <dt>This version:

    <dd><a class=u-url href="http://dev.w3.org/csswg/css3-conditional/">
     http://www.w3.org/TR/2013/ED-css3-conditional-20130608/</a>

    <dt>Latest version:

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

    <dt>Editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css3-conditional/">http://dev.w3.org/csswg/css3-conditional/</a>
     (<a
     href="https://dvcs.w3.org/hg/csswg/log/tip/css-conditional/Overview.src.html">change
     log</a>, <a
     href="https://dvcs.w3.org/hg/csswg/log/tip/css3-conditional/Overview.src.html">older
     change log</a>)

    <dt>Previous version:

    <dd><a href="http://www.w3.org/TR/2013/CR-css3-conditional-20130404/"
     rel=previous>http://www.w3.org/TR/2013/CR-css3-conditional-20130404/</a>

    <dt>Editors:

    <dd class="p-author h-card vcard"><a class="p-name fn u-url url"
     href="http://dbaron.org/" rel=author>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="mailto:www-style@w3.org?subject=%5Bcss3-conditional%5D%20feedback">www-style@w3.org</a>
     with subject line “<kbd>[css3-conditional] <var>… message topic
     …</var></kbd>” (<a
     href="http://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)

    <dt>Test suite:

    <dd><a
     href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/">http://test.csswg.org/suites/css3-conditional/nightly-unstable/</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2013 <a href="http://www.w3.org/"><abbr
    title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
    href="http://www.csail.mit.edu/"><abbr
    title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
    href="http://www.ercim.eu/"><abbr
    title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>, <a
    href="http://ev.buaa.edu.cn/">Beihang</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>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 2 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS level 1
   <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main
   extensions compared to level 2 are allowing nesting of certain at-rules
   inside ‘<code class=css>@media</code>’, and the addition of the
   ‘<code class=css>@supports</code>’ rule for conditional processing.
   </span>

  <!--begin-status-->

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss3-conditional%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   “css3-conditional” in the subject, preferably like this:
   “[<!---->css3-conditional<!---->] <em>…summary of comment…</em>”

  <p>This document was produced by the <a
   href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
   the <a href="http://www.w3.org/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
   2004 W3C Patent Policy</a>. W3C maintains a <a
   href="http://www.w3.org/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
   6 of the W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>See the section <a href="#cr-exit-criteria">“CR Exit Criteria”</a>
   for details on advancing this specification to W3C Recommendation. The
   specification will remain Candidate Recommendation at least until 2
   October 2013. A <a
   href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/report/">test
   suite and implementation report</a> are under development.

  <p>See the section <a href="#changes">“Changes”</a> for changes since
   the last Working Draft.

  <p>The following features are at risk:

  <ul>
   <li>The inclusion of ‘<code class=css>@font-face</code>’ rules and
    ‘<code class=css>@keyframes</code>’ 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>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>The ‘<code class=css>@supports</code>’ 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.
  </ul>
  <!--

  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>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
    <ul class=toc>
     <li><a href="#context"><span class=secno>1.1. </span>Background</a>

     <li><a href="#placement"><span class=secno>1.2. </span>Module
      Interactions</a>

     <li><a href="#conventions"><span class=secno>1.3. </span>Document
      Conventions</a>
    </ul>

   <li><a href="#processing"><span class=secno>2. </span>Processing of
    conditional group rules</a>

   <li><a href="#contents-of"><span class=secno>3. </span>Contents of
    conditional group rules</a>

   <li><a href="#use"><span class=secno>4. </span>Placement of conditional
    group rules</a>

   <li><a href="#at-media"><span class=secno>5. </span>Media-specific style
    sheets: the ‘<code class=css>@media</
8096
code>’ rule</a>

   <li><a href="#at-supports"><span class=secno>6. </span>Feature queries:
    the ‘<code class=css>@supports</code>’ rule</a>
    <ul class=toc>
     <li><a href="#support-definition"><span class=secno>6.1.
      </span>Definition of support</a>
    </ul>

   <li><a href="#apis"><span class=secno>7. </span>APIs</a>
    <ul class=toc>
     <li><a href="#extentions-to-cssrule-interface"><span class=secno>7.1.
      </span> Extensions to the <code>CSSRule</code> interface</a>

     <li><a href="#the-cssgroupingrule-interface"><span class=secno>7.2.
      </span> The <code>CSSGroupingRule</code> interface</a>

     <li><a href="#the-cssconditionrule-interface"><span class=secno>7.3.
      </span> The <code>CSSConditionRule</code> interface</a>

     <li><a href="#the-cssmediarule-interface"><span class=secno>7.4. </span>
      The <code>CSSMediaRule</code> interface</a>

     <li><a href="#the-csssupportsrule-interface"><span class=secno>7.5.
      </span> The <code>CSSSupportsRule</code> interface</a>

     <li><a href="#the-css-interface"><span class=secno>7.6. </span> The
      <code>CSS</code> interface, and the <code title="">supports()</code>
      function</a>
    </ul>

   <li class=no-num><a href="#grammar">Grammar</a>

   <li><a href="#conformance"><span class=secno>8. </span>Conformance</a>
    <ul class=toc>
     <li><a href="#base-modules"><span class=secno>8.1. </span>Base
      Modules</a>

     <li><a href="#conformance-classes"><span class=secno>8.2.
      </span>Conformance Classes</a>

     <li><a href="#partial"><span class=secno>8.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>8.4. </span>Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>8.5. </span>CR Exit
      Criteria</a>
    </ul>

   <li><a href="#changes"><span class=secno>9. </span> Changes</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><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>
  </ul>
  <!--end-toc-->

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

  <h3 id=context><span class=secno>1.1. </span>Background</h3>

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

  <p><a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> defines
   one type of conditional group rule, the ‘<code
   class=css>@media</code>’ rule, and allows only style rules (not other
   @-rules) inside of it. The ‘<code class=css>@media</code>’ rule
   provides the ability to have media-specific style sheets, which is also
   provided by style sheet linking features such as ‘<code
   class=css>@import</code>’ and <code class=html>&lt;link&gt;</code>. The
   restrictions on the contents of ‘<code class=css>@media</code>’ 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>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>This specification also defines an additional type of conditional group
   rule, ‘<code class=css>@supports</code>’, to address author and user
   requirements.

  <p>The ‘<code class=css>@supports</code>’ 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.

  <h3 id=placement><span class=secno>1.2. </span>Module Interactions</h3>

  <p>This module replaces and extends the ‘<code class=css>@media</code>’
   rule feature defined in <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> section <var>7.2.1</var> and
   incorporates the modifications previously made non-normatively by <a
   href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> section
   <var>1</var>.

  <p>Its current definition depends on @-rules defined in <a
   href="#CSS3-FONTS" rel=biblioentry>[CSS3-FONTS]<!--{{!CSS3-FONTS}}--></a>
   and <a href="#CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, 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.

  <h3 id=conventions><span class=secno>1.3. </span>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. <a
   href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>

  <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.
  </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.

  <h2 id=processing><span class=secno>2. </span>Processing of conditional
   group rules</h2>

  <p>This specification defines some CSS @-rules, called <dfn
   id=conditional-group-rules>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.

  <div class=example>
   <p>For example, this rule:

   <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>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.

  <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 is the page box) is less than or equal to
   12cm. Thus the rule ‘<code class=css>#navigation { display: none
   }</code>’ applies whenever this style sheet is applied to print media,
   and the rule ‘<code class=css>.note { float: none }</code>’ 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 <a href="#CSS3-TRANSITIONS"
   rel=biblioentry>[CSS3-TRANSITIONS]<!--{{CSS3-TRANSITIONS}}--></a> and <a
   href="#CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>).

  <h2 id=contents-of><span class=secno>3. </span>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 id=group-rule-body>group
   rule body</dfn>, which is a block (pair of braces) containing a sequence
   of rules.

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

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

  <p class=note>Note: Style rules are defined in grammars by the
   <code>ruleset</code> production.

  <pre><dfn id="nested_statement">nested_statement</dfn>
  : ruleset | <a
   href="#media"><i>media</i></a> | page | font_face_rule | keyframes_rule |
    <a
   href="#supports_rule"><i>supports_rule</i></a>
  ;

<dfn
   id="group_rule_body">group_rule_body</dfn>
  : '{' S* <a
   href="#nested_statement"><i>nested_statement</i></a>* '}' S*
  ;</pre>

  <p> in which all the productions are defined in that grammar with the
   exception of <code>font_face_rule</code> defined in <a href="#CSS3-FONTS"
   rel=biblioentry>[CSS3-FONTS]<!--{{!CSS3-FONTS}}--></a>,
   <code>keyframes_rule</code> defined in <a href="#CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, and <a
   href="#media"><code>media</code></a> and <a
   href="#supports_rule"><code>supports_rule</code></a> defined in this
   specification.

  <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 <a
   href="#nested_statement"><code>nested_statement</code></a> production to
   keep the grammar accurate.

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

  <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 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.

  <h2 id=use><span class=secno>4. </span>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>Any rules that are not allowed after a style rule (e.g., ‘<code
   class=css>@charset</code>’, ‘<code class=css>@import</code>’, or
   ‘<code class=css>@namespace</code>’ 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.

  <h2 id=at-media><span class=secno>5. </span>Media-specific style sheets:
   the ‘<code class=css>@media</code>’ rule</h2>

  <p>The <dfn id=atmedia-rule>‘<code class=css>@media</code>’ rule</dfn>
   is a conditional group rule whose condition is a media query. It consists
   of the at-keyword ‘<code class=css>@media</code>’ followed by a
   (possibly empty) media query list (as defined in <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>), followed by a group rule
   body. The condition of the rule is the result of the media query.

  <div class=example>
   <p>This ‘<code class=css>@media</code>’ rule:

   <pre>@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}</pre>

   <p>has the condition ‘<code class=css>screen and (min-width: 35em),
    print and (min-width: 40em)</code>’, 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
    ‘<code class=css>#section_navigation { float: left; width: 10em;
    }</code>’ is applied.
  </div>

  <p>In terms of the grammar, this specification extends the <a
   href="#media"><code>media</code></a> production in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G)
   into:

  <pre><dfn id=media>media</dfn>
  : MEDIA_SYM S* media_query_list <a
   href="#group_rule_body"><i>group_rule_body</i></a>
  ;</pre>

  <p>where the <a href="#group_rule_body"><code>group_rule_body</code></a>
   production is defined in this specification, the
   <code>media_query_list</code> production is defined in <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>, and the others are defined
   in the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS
   2.1</a> (<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>,
   Appendix G).

  <h2 id=at-supports><span class=secno>6. </span>Feature queries: the
   ‘<code class=css>@supports</code>’ rule</h2>

  <p>The <dfn id=atsupports-rule>‘<code class=css>@supports</code>’
   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>The syntax of the condition in the ‘<code
   class=css>@supports</code>’ rule is slightly more complicated than for
   the other conditional group rules (though has some similarities to media
   queries) since:

  <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>conjunction (and) is needed so that multiple required features can be
    tested

   <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
  </ul>

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

  <p>This extends the lexical scanner in the <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by
   adding:

  <pre>
@{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn
   id="supports_sym">SUPPORTS_SYM</dfn>;}
{O}{R}                    {return <dfn
   id=or>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 <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> and the <a
   href="#or"><code>OR</code></a> and <a
   href="#supports_sym"><code>SUPPORTS_SYM</code></a> 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>
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1
   (Tokenization)</a> of <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, by adding:

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

<dfn
   id="supports_condition">supports_condition</dfn>
  : <a
   href="#supports_negation"><i>supports_negation</i></a> | <a
   href="#supports_conjunction"><i>supports_conjunction</i></a> | <a
   href="#supports_disjunction"><i>supports_disjunction</i></a> |
    <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a>
  ;

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

<dfn
   id="supports_negation">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>+ <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a>
  ;

<dfn
   id="supports_conjunction">supports_conjunction</dfn>
  : <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a> ( <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>+ <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a> )+
  ;

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

<dfn
   id="supports_declaration_condition">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
   id="general_enclosed">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 ‘<code
   class=css>@supports</code>’ 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> The above grammar is purposely very loose for forwards-compatibility
   reasons, since the <a
   href="#general_enclosed"><code>general_enclosed</code></a> production
   allows for substantial future extensibility. Any ‘<code
   class=css>@supports</code>’ rule 
AA29
that does not parse according to the
   grammar above (that is, a rule that does not match this loose grammar
   which includes the <a
   href="#general_enclosed"><code>general_enclosed</code></a> 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:

  <dl>
   <dt>supports_condition

   <dd> The result is the result of the single child term.

   <dt>supports_condition_in_parens

   <dd> The result is the result of the single <a
    href="#supports_condition"><code>supports_condition</code></a> or <a
    href="#supports_declaration_condition"><code>supports_declaration_condition</code></a>
    child term.

   <dt>supports_negation

   <dd> The result is the <em>negation</em> of the result of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child term.

   <dt>supports_conjunction

   <dd> The result is true if the result of <em>all</em> of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child terms is true; otherwise it is false.

   <dt>supports_disjunction

   <dd> The result is true if the result of <em>any</em> of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child terms is true; otherwise it is false.

   <dt>supports_declaration_condition

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

   <dt>general_enclosed

   <dd> The result is always false. Additionally, style sheets <strong>must
    not</strong> write ‘<code class=css>@supports</code>’ 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>
  </dl>

  <p>The condition of the ‘<code class=css>@supports</code>’ rule is the
   result of the <a
   href="#supports_condition"><code>supports_condition</code></a> term that
   is a child of the <a href="#supports_rule"><code>supports_rule</code></a>
   term.

  <div class=example>
   <p>For example, the following rule

   <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 ‘<code class=css>@supports</code>’
    rule only when ‘<code class=css>display: flexbox</code>’ is
    supported.
  </div>

  <div class=example>
   <p>The following example shows an additional ‘<code
    class=css>@supports</code>’ rule that can be used to provide an
    alternative for when ‘<code class=css>display: flexbox</code>’ is not
    supported:

   <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 ‘<code class=property>width</code>’ declarations may
    be harmful to the flexbox-based layout, so it is important that they be
    present only in the non-flexbox styles.
  </div>

  <div class=example>
   <p>The following example checks for support for the ‘<code
    class=property>box-shadow</code>’ property, including checking for
    support for vendor-prefixed versions of it. When the support is present,
    it specifies both ‘<code class=property>box-shadow</code>’ (with the
    prefixed versions) and ‘<code class=property>border</code>’ in a way
    what would cause the box to become invisible were ‘<code
    class=property>box-shadow</code>’ not supported.

   <pre>.noticebox {
  border: 1px solid black;
  padding: 1px;
}
@supports ( box-shadow: 0 0 2px black inset ) or
          ( -moz-box-shadow: 0 0 2px black inset ) or
          ( -webkit-box-shadow: 0 0 2px black inset ) or
          ( -o-box-shadow: 0 0 2px black inset ) {
  .noticebox {
    -moz-box-shadow: 0 0 2px black inset;
    -webkit-box-shadow: 0 0 2px black inset;
    -o-box-shadow: 0 0 2px black inset;
    box-shadow: 0 0 2px black inset; /* unprefixed last */
    /* override the rule above the @supports rule */
    border: none;
    padding: 2px;
  }
}</pre>
  </div>

  <p>To avoid confusion between ‘<code class=css>and</code>’ and ‘<a
   href="#or"><code class=css>or</code></a>’, the syntax requires that both
   ‘<code class=css>and</code>’ and ‘<a href="#or"><code
   class=css>or</code></a>’ 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 ‘<code
   class=css>and</code>’, ‘<a href="#or"><code
   class=css>or</code></a>’, and ‘<code class=css>not</code>’ operators
   to be mixed without a layer of parentheses.

  <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:

   <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 ‘<code
   class=css>not</code>’ and on both sides of an ‘<code
   class=css>and</code>’ or ‘<a href="#or"><code
   class=css>or</code></a>’.

  <p>The declaration being tested must always occur within parentheses, when
   it is the only thing in 
6197
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:

   <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.

  <div class=example>
   <p>For example, authors may write:

   <pre>@supports ((display: flexbox)) {
  // ...
}</pre>
  </div>

  <p>A trailing ‘<code class=css>!important</code>’ 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><span class=secno>6.1. </span>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 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> 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>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 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 ‘<code class=property>color</code>’ property even though
   declarations of the ‘<code class=property>color</code>’ 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>These rules (and the equivalence between them) allow authors to use
   fallback (either in the <a href="#CSS1"
   rel=biblioentry>[CSS1]<!--{{CSS1}}--></a> sense of declarations that are
   overridden by later declarations or with the new capabilities provided by
   the ‘<code class=css>@supports</code>’ 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 style rule
   or in the declaration condition of an ‘<code
   class=css>@supports</code>’ 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]]'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]]'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