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Overview.html
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<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>CSS Conditional Rules Module Level 3</title>
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<meta content="testing" name="csswg-work-status">
<meta content="ED" name="w3c-status">
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
body {
background-image: url("https://www.w3.org/StyleSheets/TR/logo-ED");
};
</style>
<meta content="Bikeshed 1.0.0" name="generator">
<style>.css::before, .css::after, .property::before, .property::after { content: none !important;}</style>
<style>/* style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
regarding paragraph and lists. */
[data-md] > :first-child {
margin-top: 0;
}
[data-md] > :last-child {
margin-bottom: 0;
}</style>
<style>/* style-counters */
.issue:not(.no-marker)::before {
content: "Issue " counter(issue);
}
.example:not(.no-marker)::before {
content: "Example";
content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
content: "Invalid Example";
content: "Invalid Example" counter(example);
}</style>
<style>/* style-dfn-panel */
.dfn-panel {
display: inline-block;
position: absolute;
z-index: 35;
height: auto;
width: -webkit-fit-content;
max-width: 300px;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.75em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: #DDDDDD;
color: black;
border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel > span { display: list-item; list-style: inside; }
.dfn-panel.activated {
display: inline-block;
position: fixed;
left: .5em;
bottom: .5em;
margin: 0 auto;
max-width: calc(100vw - 1.5em - .4em - .5em);
max-height: 30vh;
}
.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-selflinks */
.heading, .issue, .note, .example, li, dt {
position: relative;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
}
a.self-link:hover {
opacity: 1;
}
.heading > a.self-link {
font-size: 83%;
}
li > a.self-link {
left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
top: auto;
left: auto;
opacity: 0;
width: 1.5em;
height: 1.5em;
background: gray;
color: white;
font-style: normal;
transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
opacity: 1;
}
dfn > a.self-link:hover {
color: black;
}
a.self-link::before { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before { content: "#"; }</style>
<style>/* style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
color: #005a9c;
font-size: inherit;
font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
content: "‘";
}
.css::after, .property::after, .descriptor::after {
content: "’";
}
.property, .descriptor {
/* Don't wrap property and descriptor names */
white-space: nowrap;
}
.type { /* CSS value <type> */
font-style: italic;
}
pre .property::before, pre .property::after {
content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
content: "’";
}
[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
content: "";
}
[data-link-type=element],
[data-link-type=element-attr] {
font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after { content: ">" }
[data-link-type=biblio] {
white-space: pre;
}</style>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">CSS Conditional Rules Module Level 3</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft, <time class="dt-updated" datetime="2016-05-13">13 May 2016</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://drafts.csswg.org/css3-conditional/">https://drafts.csswg.org/css3-conditional/</a>
<dt>Latest published version:
<dd><a href="https://www.w3.org/TR/css3-conditional/">https://www.w3.org/TR/css3-conditional/</a>
<dt>Previous Versions:
<dd><a href="https://www.w3.org/TR/2013/CR-css3-conditional-20130404/" rel="previous">https://www.w3.org/TR/2013/CR-css3-conditional-20130404/</a>
<dt>Feedback:
<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss3-conditional%5D%20YOUR%20TOPIC%20HERE">www-style@w3.org</a> with subject line “<kbd>[css3-conditional] <i data-lt="">… message topic …</i></kbd>” (<a href="http://lists.w3.org/Archives/Public/www-style/" rel="discussion">archives</a>)</span>
<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>
<dt class="editor">Editor:
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://dbaron.org">L. David Baron</a> (<a class="p-org org" href="http://www.mozilla.org">Mozilla</a>)
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2016 <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>). 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>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<p>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 data-link-type="biblio" href="#biblio-css21">[CSS21]</a>, which builds on CSS level 1 <a data-link-type="biblio" href="#biblio-css1">[CSS1]</a>. The main extensions compared to level 2 are allowing nesting of
certain at-rules inside '@media', and the addition of the '@supports' rule for
conditional processing.</p>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.
</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="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>
<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>
<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>
<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>
<p> This document is governed by the <a href="http://www.w3.org/2015/Process-20150901/" id="w3c_process_revision">1 September 2015 W3C Process Document</a>. </p>
<p></p>
</div>
<div data-fill-with="at-risk">
<p>The following features are at-risk, and may be dropped during the CR period: </p>
<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>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 @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.
</ul>
<p>“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.</p>
</div>
<nav data-fill-with="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li>
<a href="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
<ol class="toc">
<li><a href="#context"><span class="secno">1.1</span> <span class="content">Background</span></a>
<li><a href="#placement"><span class="secno">1.2</span> <span class="content">Module Interactions</span></a>
</ol>
<li><a href="#processing"><span class="secno">2</span> <span class="content">Processing of conditional group rules</span></a>
<li><a href="#contents-of"><span class="secno">3</span> <span class="content">Contents of conditional group rules</span></a>
<li><a href="#use"><span class="secno">4</span> <span class="content">Placement of conditional group rules</span></a>
<li><a href="#at-media"><span class="secno">5</span> <span class="content">Media-specific style sheets: the '@media' rule</span></a>
<li>
<a href="#at-supports"><span class="secno">6</span> <span class="content">Feature queries: the '@supports' rule</span></a>
<ol class="toc">
<li><a href="#support-definition"><span class="secno">6.1</span> <span class="content">Definition of support</span></a>
</ol>
<li>
<a href="#apis"><span class="secno">7</span> <span class="content">APIs</span></a>
<ol class="toc">
<li><a href="#extentions-to-cssrule-interface"><span class="secno">7.1</span> <span class="content"> Extensions to the <code>CSSRule</code> interface</span></a>
<li><a href="#the-cssgroupingrule-interface"><span class="secno">7.2</span> <span class="content"> The <code>CSSGroupingRule</code> interface</span></a>
<li><a href="#the-cssconditionrule-interface"><span class="secno">7.3</span> <span class="content"> The <code>CSSConditionRule</code> interface</span></a>
<li><a href="#the-cssmediarule-interface"><span class="secno">7.4</span> <span class="content"> The <code>CSSMediaRule</code> interface</span></a>
<li><a href="#the-csssupportsrule-interface"><span class="secno">7.5</span> <span class="content"> The <code>CSSSupportsRule</code> interface</span></a>
<li><a href="#the-css-interface"><span class="secno">7.6</span> <span class="content"> The <code>CSS</code> interface, and the <code title="">supports()</code> function</span></a>
</ol>
<li><a href="#changes"><span class="secno">8</span> <span class="content"> Changes</span></a>
<li><a href="#acknowledgments"><span class="secno"></span> <span class="content">Acknowledgments</span></a>
<li>
<a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
<ol class="toc">
<li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
<li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
<li>
<a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
<ol class="toc">
<li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
<li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
<li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
</ol>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
<li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
</ol>
<li>
<a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ol>
<li><a href="#idl-index"><span class="secno"></span> <span class="content">IDL Index</span></a>
</ol>
</nav>
<main>
<h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
<h3 class="heading settled" data-level="1.1" id="context"><span class="secno">1.1. </span><span class="content">Background</span><a class="self-link" href="#context"></a></h3>
<p><em>This section is not normative.</em> </p>
<p><a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> defines one type of conditional group rule, the
'@media' rule, and allows only style rules (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"><link></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 class="heading settled" data-level="1.2" id="placement"><span class="secno">1.2. </span><span class="content">Module Interactions</span><a class="self-link" href="#placement"></a></h3>
<p>This module replaces and extends the '@media' rule
feature defined in <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> section <var>7.2.1</var> and
incorporates the modifications previously made non-normatively by <a data-link-type="biblio" href="#biblio-mediaq">[MEDIAQ]</a> section <var>1</var>.</p>
<p>Its current definition depends on @-rules defined in <a data-link-type="biblio" href="#biblio-css3-fonts">[CSS3-FONTS]</a> and <a data-link-type="biblio" href="#biblio-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.</p>
<p>In order to allow these new @-rules in CSS style sheets, this
specification modifies the <code>stylesheet</code> production in the <a href="https://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> by replacing the <code>media</code> production defined in <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a> with the <code>media</code> production defined in this one,
and additionally inserting <code>| supports_rule</code> alongside <code>ruleset | media | page</code>.</p>
<h2 class="heading settled" data-level="2" id="processing"><span class="secno">2. </span><span class="content">Processing of conditional group rules</span><a class="self-link" href="#processing"></a></h2>
<p>This specification defines some CSS @-rules, called <dfn data-dfn-type="dfn" data-noexport="" id="conditional-group-rules">conditional
group rules<a class="self-link" href="#conditional-group-rules"></a></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" id="example-74177fce">
<a class="self-link" href="#example-74177fce"></a>
<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. </p>
</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" id="example-225874d2">
<a class="self-link" href="#example-225874d2"></a>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>
<p>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 <span class="css">#navigation { display: none }</span> applies
whenever this style sheet is applied to print media, and the rule <span class="css">.note { float: none }</span> 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.</p>
</div>
<p></p>
<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 data-link-type="biblio" href="#biblio-css3-transitions">[CSS3-TRANSITIONS]</a> and <a data-link-type="biblio" href="#biblio-css3-animations">[CSS3-ANIMATIONS]</a>).</p>
<h2 class="heading settled" data-level="3" id="contents-of"><span class="secno">3. </span><span class="content">Contents of conditional group rules</span><a class="self-link" href="#contents-of"></a></h2>
<p>The syntax of each conditional group rule consists of some syntax
specific to the type of rule followed by a <dfn data-dfn-type="dfn" data-noexport="" id="group-rule-body">group rule body<a class="self-link" href="#group-rule-body"></a></dfn>,
which is a block (pair of braces) containing a sequence of rules.</p>
<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 '@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>
<p class="note" role="note">Note: Style rules are defined in grammars
by the <code>ruleset</code> production.</p>
<pre><dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="nested_statement" data-noexport="" id="nested_statement">nested_statement<span class="dfn-panel" data-deco=""><b><a href="#nested_statement">#nested_statement</a></b><b>Referenced in:</b><span><a href="#ref-for-nested_statement-1">3. Contents of conditional group rules</a></span></span></dfn>
: ruleset | <a data-link-type="dfn" href="#media" id="ref-for-media-1">media</a> | page | font_face_rule | keyframes_rule |
<a data-link-type="dfn" href="#supports_rule" id="ref-for-supports_rule-1">supports_rule</a>
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="group_rule_body" data-noexport="" id="group_rule_body">group_rule_body<span class="dfn-panel" data-deco=""><b><a href="#group_rule_body">#group_rule_body</a></b><b>Referenced in:</b><span><a href="#ref-for-group_rule_body-1">5. Media-specific style sheets: the '@media' rule</a></span><span><a href="#ref-for-group_rule_body-2">6. Feature queries: the '@supports' rule</a></span></span></dfn>
: '{' S* <a data-link-type="dfn" href="#nested_statement" id="ref-for-nested_statement-1">nested_statement</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 data-link-type="biblio" href="#biblio-css3-fonts">[CSS3-FONTS]</a>, <code>keyframes_rule</code> defined in <a data-link-type="biblio" href="#biblio-css3-animations">[CSS3-ANIMATIONS]</a>, 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="https://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
4.2 (Rules for handling parsing errors)</a>, <a href="https://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
(At-rules)</a>, and <a href="https://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
(Rule sets, declaration blocks, and selectors)</a> of <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>.</p>
<h2 class="heading settled" data-level="4" id="use"><span class="secno">4. </span><span class="content">Placement of conditional group rules</span><a class="self-link" href="#use"></a></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 style rule (e.g., <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-syntax-3/#at-ruledef-charset">@charset</a>, <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-cascade-4/#at-ruledef-import">@import</a>, or <span class="css">@namespace</span> 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 class="heading settled" data-level="5" id="at-media"><span class="secno">5. </span><span class="content">Media-specific style sheets: the '@media' rule</span><a class="self-link" href="#at-media"></a></h2>
<p>The <dfn class="dfn-paneled css" data-dfn-type="at-rule" data-export="" data-lt="@media" id="at-ruledef-media">@media<span class="dfn-panel" data-deco=""><b><a href="#at-ruledef-media">#at-ruledef-media</a></b><b>Referenced in:</b><span><a href="#ref-for-at-ruledef-media-1">7.4.
The CSSMediaRule interface</a> <a href="#ref-for-at-ruledef-media-2">(2)</a></span></span></dfn> rule 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 <a data-link-type="biblio" href="#biblio-mediaq">[MEDIAQ]</a>), followed by a group rule body. The condition
of the rule is the result of the media query.</p>
<div class="example" id="example-b54b3e7d">
<a class="self-link" href="#example-b54b3e7d"></a>
<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 <span class="css">screen and (min-width: 35em), print and (min-width: 40em)</span>,
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 <span class="css">#section_navigation { float: left; width: 10em; }</span> is applied.</p>
</div>
<p>In terms of the grammar, this specification extends the <code>media</code> production in the <a href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>, Appendix G) into: </p>
<pre><dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="media" data-noexport="" id="media">media<span class="dfn-panel" data-deco=""><b><a href="#media">#media</a></b><b>Referenced in:</b><span><a href="#ref-for-media-1">3. Contents of conditional group rules</a></span></span></dfn>
: MEDIA_SYM S* media_query_list <a data-link-type="dfn" href="#group_rule_body" id="ref-for-group_rule_body-1">group_rule_body</a>
;
</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 <a data-link-type="biblio" href="#biblio-mediaq">[MEDIAQ]</a>, and the others are defined in the <a href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>, Appendix G). </p>
<h2 class="heading settled" data-level="6" id="at-supports"><span class="secno">6. </span><span class="content">Feature queries: the '@supports' rule</span><a class="self-link" href="#at-supports"></a></h2>
<p>The <dfn class="dfn-paneled css" data-dfn-type="at-rule" data-export="" data-lt="@supports" id="at-ruledef-supports">@supports<span class="dfn-panel" data-deco=""><b><a href="#at-ruledef-supports">#at-ruledef-supports</a></b><b>Referenced in:</b><span><a href="#ref-for-at-ruledef-supports-1">6. Feature queries: the '@supports' rule</a> <a href="#ref-for-at-ruledef-supports-2">(2)</a> <a href="#ref-for-at-ruledef-supports-3">(3)</a></span><span><a href="#ref-for-at-ruledef-supports-4">6.1. Definition of support</a> <a href="#ref-for-at-ruledef-supports-5">(2)</a></span><span><a href="#ref-for-at-ruledef-supports-6">7.5.
The CSSSupportsRule interface</a></span></span></dfn> rule 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>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 '@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="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>, Appendix G) by adding: </p>
<pre>@{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="SUPPORTS_SYM" data-noexport="" id="supports_sym">SUPPORTS_SYM<span class="dfn-panel" data-deco=""><b><a href="#supports_sym">#supports_sym</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_sym-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>;}
{O}{R} {return <dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="OR" data-noexport="" id="or">OR<span class="dfn-panel" data-deco=""><b><a href="#or">#or</a></b><b>Referenced in:</b><span><a href="#ref-for-or-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>;}
</pre>
<p>This then extends the grammar in the <a href="https://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="https://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and <code><a href="https://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code> tokens defined in the Media Queries specification <a data-link-type="biblio" href="#biblio-mediaq">[MEDIAQ]</a> and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
and with <code><a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>, <code><a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
and <code><a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code> productions
and the <code><a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
taken from the core syntax of CSS defined in <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of <a data-link-type="biblio" href="#biblio-css21">[CSS21]</a>,
by adding:</p>
<pre><dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_rule" data-noexport="" id="supports_rule">supports_rule<span class="dfn-panel" data-deco=""><b><a href="#supports_rule">#supports_rule</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_rule-1">3. Contents of conditional group rules</a></span></span></dfn>
: <a data-link-type="dfn" href="#supports_sym" id="ref-for-supports_sym-1">SUPPORTS_SYM</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a data-link-type="dfn" href="#supports_condition" id="ref-for-supports_condition-1">supports_condition</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a data-link-type="dfn" href="#group_rule_body" id="ref-for-group_rule_body-2">group_rule_body</a>
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_condition" data-noexport="" id="supports_condition">supports_condition<span class="dfn-panel" data-deco=""><b><a href="#supports_condition">#supports_condition</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_condition-1">6. Feature queries: the '@supports' rule</a> <a href="#ref-for-supports_condition-2">(2)</a></span></span></dfn>
: <a data-link-type="dfn" href="#supports_negation" id="ref-for-supports_negation-1">supports_negation</a> | <a data-link-type="dfn" href="#supports_conjunction" id="ref-for-supports_conjunction-1">supports_conjunction</a> | <a data-link-type="dfn" href="#supports_disjunction" id="ref-for-supports_disjunction-1">supports_disjunction</a> |
<a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-1">supports_condition_in_parens</a>
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_condition_in_parens" data-noexport="" id="supports_condition_in_parens">supports_condition_in_parens<span class="dfn-panel" data-deco=""><b><a href="#supports_condition_in_parens">#supports_condition_in_parens</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_condition_in_parens-1">6. Feature queries: the '@supports' rule</a> <a href="#ref-for-supports_condition_in_parens-2">(2)</a> <a href="#ref-for-supports_condition_in_parens-3">(3)</a> <a href="#ref-for-supports_condition_in_parens-4">(4)</a> <a href="#ref-for-supports_condition_in_parens-5">(5)</a> <a href="#ref-for-supports_condition_in_parens-6">(6)</a></span></span></dfn>
: ( '(' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a data-link-type="dfn" href="#supports_condition" id="ref-for-supports_condition-2">supports_condition</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* ')' ) | <a data-link-type="dfn" href="#supports_declaration_condition" id="ref-for-supports_declaration_condition-1">supports_declaration_condition</a> |
<a data-link-type="dfn" href="#general_enclosed" id="ref-for-general_enclosed-1">general_enclosed</a>
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_negation" data-noexport="" id="supports_negation">supports_negation<span class="dfn-panel" data-deco=""><b><a href="#supports_negation">#supports_negation</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_negation-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>
: <a href="https://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>+ <a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-2">supports_condition_in_parens</a>
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_conjunction" data-noexport="" id="supports_conjunction">supports_conjunction<span class="dfn-panel" data-deco=""><b><a href="#supports_conjunction">#supports_conjunction</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_conjunction-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>
: <a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-3">supports_condition_in_parens</a> ( <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>+ <a href="https://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>+ <a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-4">supports_condition_in_parens</a> )+
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_disjunction" data-noexport="" id="supports_disjunction">supports_disjunction<span class="dfn-panel" data-deco=""><b><a href="#supports_disjunction">#supports_disjunction</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_disjunction-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>
: <a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-5">supports_condition_in_parens</a> ( <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>+ <a data-link-type="dfn" href="#or" id="ref-for-or-1">OR</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>+ <a data-link-type="dfn" href="#supports_condition_in_parens" id="ref-for-supports_condition_in_parens-6">supports_condition_in_parens</a> )+
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="supports_declaration_condition" data-noexport="" id="supports_declaration_condition">supports_declaration_condition<span class="dfn-panel" data-deco=""><b><a href="#supports_declaration_condition">#supports_declaration_condition</a></b><b>Referenced in:</b><span><a href="#ref-for-supports_declaration_condition-1">6. Feature queries: the '@supports' rule</a></span></span></dfn>
: '(' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a> ')'
;
<dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="general_enclosed" data-noexport="" id="general_enclosed">general_enclosed<span class="dfn-panel" data-deco=""><b><a href="#general_enclosed">#general_enclosed</a></b><b>Referenced in:</b><span><a href="#ref-for-general_enclosed-1">6. Feature queries: the '@supports' rule</a></span><span><a href="#ref-for-general_enclosed-2">7.5.
The CSSSupportsRule interface</a></span></span></dfn>
: ( <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a> | '(' ) ( <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a> | <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a> )* ')'
;
</pre>
<p>The <dfn class="css" data-dfn-type="type" data-export="" id="typedef-supports-condition"><supports-condition><a class="self-link" href="#typedef-supports-condition"></a></dfn> production is defined as matching the <code>supports_condition</code> grammar, defined above.</p>
<p> Implementations <strong>must</strong> parse <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-1">@supports</a> 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 <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-2">@supports</a> 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>
<p>Each of these grammar terms is associated with a boolean result, as
follows:</p>
<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 <code>supports_condition</code> or <code>supports_declaration_condition</code> child term.
<dt>supports_negation
<dd> The result is the <em>negation</em> of the result of the <code>supports_condition_in_parens</code> child term.
<dt>supports_conjunction
<dd> The 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.
<dt>supports_disjunction
<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.
<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 <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-3">@supports</a> 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" role="note">Note that future levels may define functions
or other parenthesized expressions that can evaluate to true.</span>
</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" id="example-4c43710a">
<a class="self-link" href="#example-4c43710a"></a>
<p>For example, the following rule</p>
<pre>@supports ( display: flex ) {
body, #navigation, #content { display: flex; }
#navigation { background: blue; color: white; }
#article { background: white; color: black; }
}
</pre>
<p>applies the rules inside the '@supports' rule only when <a class="css" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visuren.html#propdef-display">display: flex</a> is supported.</p>
</div>
<div class="example" id="example-1ecde2ae">
<a class="self-link" href="#example-1ecde2ae"></a>
<p>The following example shows an additional '@supports' rule that can
be used to provide an alternative for when <a class="css" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visuren.html#propdef-display">display: flex</a> is not
supported:</p>
<pre>@supports not ( display: flex ) {
body { width: 100%; height: 100%; background: white; color: black; }
#navigation { width: 25%; }
#article { width: 75%; }
}
</pre>
<p>Note that the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css2/visudet.html#propdef-width">width</a> declarations may be harmful to the
flex-based layout, so it is important that they be present only in
the non-flex styles.</p>
</div>
<div class="example" id="example-6a3345d1">
<a class="self-link" href="#example-6a3345d1"></a>
<p>The following example checks for support for the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-backgrounds-3/#box-shadow">box-shadow</a> property, including checking for support for vendor-prefixed versions of
it. When the support is present, it specifies both <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-backgrounds-3/#box-shadow">box-shadow</a> (with
the prefixed versions) and <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-backgrounds-3/#border">border</a> in a way what would cause the box to
become invisible were <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-backgrounds-3/#box-shadow">box-shadow</a> not supported.</p>
<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 <span class="css">and</span> and <span class="css">or</span>, the syntax requires
that both <span class="css">and</span> and <span class="css">or</span> 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 <span class="css">and</span>, <span class="css">or</span>,
and <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/mediaqueries-4/#valdef-media-not">not</a> operators to be mixed without a layer of parentheses.</p>
<div class="example" id="example-00e4ce41">
<a class="self-link" href="#example-00e4ce41"></a>
<p>For example, the following rule is not valid: </p>
<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 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/mediaqueries-4/#valdef-media-not">not</a> and on both
sides of an <span class="css">and</span> or <span class="css">or</span>.</p>
<p>The declaration being tested must always occur within parentheses,
when it is the only thing in the expression.</p>
<p></p>
<div class="example" id="example-1cfe6d41">
<a class="self-link" href="#example-1cfe6d41"></a>
<p>For example, the following rule is not valid: </p>
<pre class="illegal">@supports display: flex {
// ...
}
</pre>
<p>Instead, authors must write:</p>
<pre>@supports (display: flex) {
// ...
}
</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" id="example-cbfc6539">
<a class="self-link" href="#example-cbfc6539"></a>
<p>For example, authors may write:</p>
<pre>@supports ((display: flex)) {
// ...
}
</pre>
</div>
<p>A trailing <span class="css">!important</span> on a declaration being tested is allowed,
though it won’t change the validity of the declaration. </p>
<div class="example" id="example-8a8943b5">
<a class="self-link" href="#example-8a8943b5"></a>
<p>For example, the following rule is valid: </p>
<pre>@supports (display: flex !important) {
// ...
}
</pre>
</div>
<h3 class="heading settled" data-level="6.1" id="support-definition"><span class="secno">6.1. </span><span class="content">Definition of support</span><a class="self-link" href="#support-definition"></a></h3>
<p>For forward-compatibility, <a href="https://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
(Declarations and properties)</a> of <a data-link-type="biblio" href="#biblio-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>
<p>A CSS processor is considered to <dfn data-dfn-type="dfn" data-noexport="" id="dfn-support">support<a class="self-link" href="#dfn-support"></a></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" role="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 <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-color-3/#color0">color</a> property
even though declarations of the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-color-3/#color0">color</a> 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 <a data-link-type="biblio" href="#biblio-css1">[CSS1]</a> sense of declarations
that are overridden by later declarations or with the new capabilities
provided by the <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-4">@supports</a> 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 <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-5">@supports</a> rule.</p>
<h2 class="heading settled" data-level="7" id="apis"><span class="secno">7. </span><span class="content">APIs</span><a class="self-link" href="#apis"></a></h2>
<h3 class="heading settled" data-level="7.1" id="extentions-to-cssrule-interface"><span class="secno">7.1. </span><span class="content"> Extensions to the <code>CSSRule</code> interface</span><a class="self-link" href="#extentions-to-cssrule-interface"></a></h3>
<p>The <code>CSSRule</code> interface is extended as follows: </p>
<pre class="idl def">partial interface <a class="idl-code" data-link-type="interface" href="https://drafts.csswg.org/cssom-1/#cssrule">CSSRule</a> {
const unsigned short <dfn class="idl-code" data-dfn-for="CSSRule" data-dfn-type="const" data-export="" id="dom-cssrule-supports_rule">SUPPORTS_RULE<a class="self-link" href="#dom-cssrule-supports_rule"></a></dfn> = 12;
};
</pre>
<h3 class="heading settled" data-level="7.2" id="the-cssgroupingrule-interface"><span class="secno">7.2. </span><span class="content"> The <code>CSSGroupingRule</code> interface</span><a class="self-link" href="#the-cssgroupingrule-interface"></a></h3>
<p>The <code class="idl"><a data-link-type="idl" href="#cssgroupingrule" id="ref-for-cssgroupingrule-1">CSSGroupingRule</a></code> interface represents an at-rule that contains other rules nested inside itself. </p>
<pre class="idl def">interface <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export="" data-lt="CSSGroupingRule" id="cssgroupingrule">CSSGroupingRule<span class="dfn-panel" data-deco=""><b><a href="#cssgroupingrule">#cssgroupingrule</a></b><b>Referenced in:</b><span><a href="#ref-for-cssgroupingrule-1">7.2.
The CSSGroupingRule interface</a></span><span><a href="#ref-for-cssgroupingrule-2">7.3.
The CSSConditionRule interface</a></span></span></dfn> : <a data-link-type="idl-name" href="https://drafts.csswg.org/cssom-1/#cssrule">CSSRule</a> {
readonly attribute <a data-link-type="idl-name" href="https://drafts.csswg.org/cssom-1/#cssrulelist">CSSRuleList</a> <a class="idl-code" data-link-type="attribute" data-readonly="" data-type="CSSRuleList" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-cssrules">cssRules</a>;
unsigned long <a class="idl-code" data-link-type="method" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-insertrule">insertRule</a> (DOMString <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-insertrule-rule-index-rule">rule</a>, unsigned long <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-insertrule-rule-index-index">index</a>);
void <a class="idl-code" data-link-type="method" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-deleterule">deleteRule</a> (unsigned long <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/cssom-1/#dom-cssgroupingrule-deleterule-index-index">index</a>);
};
</pre>
<dl class="idl-attributes">
<dt><code>cssRules</code> of type <code><a href="https://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 <var>rule</var> 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 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-cascade-4/#at-ruledef-import">@import</a> rule is inserted inside a group rule.
<p>The return value is the <var>index</var> parameter. </p>
<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 class="heading settled" data-level="7.3" id="the-cssconditionrule-interface"><span class="secno">7.3. </span><span class="content"> The <code>CSSConditionRule</code> interface</span><a class="self-link" href="#the-cssconditionrule-interface"></a></h3>
<p>The <code class="idl"><a data-link-type="idl" href="#cssconditionrule" id="ref-for-cssconditionrule-1">CSSConditionRule</a></code> interface represents
all the “conditional” at-rules,
which consist of a condition and a statement block. </p>
<pre class="idl def">interface <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export="" data-lt="CSSConditionRule" id="cssconditionrule">CSSConditionRule<span class="dfn-panel" data-deco=""><b><a href="#cssconditionrule">#cssconditionrule</a></b><b>Referenced in:</b><span><a href="#ref-for-cssconditionrule-1">7.3.
The CSSConditionRule interface</a></span><span><a href="#ref-for-cssconditionrule-2">7.4.
The CSSMediaRule interface</a></span><span><a href="#ref-for-cssconditionrule-3">7.5.
The CSSSupportsRule interface</a></span></span></dfn> : <a data-link-type="idl-name" href="#cssgroupingrule" id="ref-for-cssgroupingrule-2">CSSGroupingRule</a> {
attribute DOMString <dfn class="idl-code" data-dfn-for="CSSConditionRule" data-dfn-type="attribute" data-export="" data-type="DOMString" id="dom-cssconditionrule-conditiontext">conditionText<a class="self-link" href="#dom-cssconditionrule-conditiontext"></a></dfn>;
};
</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>
<p>On setting the <code>conditionText</code> attribute these steps
must be run: </p>
<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 class="heading settled" data-level="7.4" id="the-cssmediarule-interface"><span class="secno">7.4. </span><span class="content"> The <code>CSSMediaRule</code> interface</span><a class="self-link" href="#the-cssmediarule-interface"></a></h3>
<p>The <code class="idl"><a data-link-type="idl" href="#cssmediarule" id="ref-for-cssmediarule-1">CSSMediaRule</a></code> interface represents a <a class="css" data-link-type="maybe" href="#at-ruledef-media" id="ref-for-at-ruledef-media-1">@media</a> rule: </p>
<pre class="idl def">interface <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export="" data-lt="CSSMediaRule" id="cssmediarule">CSSMediaRule<span class="dfn-panel" data-deco=""><b><a href="#cssmediarule">#cssmediarule</a></b><b>Referenced in:</b><span><a href="#ref-for-cssmediarule-1">7.4.
The CSSMediaRule interface</a></span></span></dfn> : <a data-link-type="idl-name" href="#cssconditionrule" id="ref-for-cssconditionrule-2">CSSConditionRule</a> {
readonly attribute <a data-link-type="idl-name" href="https://drafts.csswg.org/cssom-1/#medialist">MediaList</a> <a class="idl-code" data-link-type="attribute" data-readonly="" data-type="MediaList" href="https://drafts.csswg.org/cssom-1/#dom-cssmediarule-media">media</a>;
};
</pre>
<dl class="idl-attributes">
<dt><code>media</code> of type <code><a href="https://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a></code>, readonly
<dd>The <code>media</code> attribute must return a <code>MediaList</code> object
for the list of media queries specified with the <a class="css" data-link-type="maybe" href="#at-ruledef-media" id="ref-for-at-ruledef-media-2">@media</a> 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. </p>
</dl>
<h3 class="heading settled" data-level="7.5" id="the-csssupportsrule-interface"><span class="secno">7.5. </span><span class="content"> The <code>CSSSupportsRule</code> interface</span><a class="self-link" href="#the-csssupportsrule-interface"></a></h3>
<p>The <code class="idl"><a data-link-type="idl" href="#csssupportsrule" id="ref-for-csssupportsrule-1">CSSSupportsRule</a></code> interface represents a <a class="css" data-link-type="maybe" href="#at-ruledef-supports" id="ref-for-at-ruledef-supports-6">@supports</a> rule.</p>
<pre class="idl def">interface <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export="" data-lt="CSSSupportsRule" id="csssupportsrule">CSSSupportsRule<span class="dfn-panel" data-deco=""><b><a href="#csssupportsrule">#csssupportsrule</a></b><b>Referenced in:</b><span><a href="#ref-for-csssupportsrule-1">7.5.
The CSSSupportsRule interface</a></span></span></dfn> : <a data-link-type="idl-name" href="#cssconditionrule" id="ref-for-cssconditionrule-3">CSSConditionRule</a> {
};
</pre>
<dl class="idl-attributes">
<dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-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 condition that was specified,
without any logical simplifications,
so that the returned condition will evaluate to the same result
as the specified condition
in any conformant implementation of this specification
(including implementations that implement future extensions
allowed by the <a data-link-type="dfn" href="#general_enclosed" id="ref-for-general_enclosed-2">general_enclosed</a> exensibility mechanism in this specification).
In other words,
token stream simplifications are allowed
(such as reducing whitespace to a single space
or omitting it in cases where it is known to be optional),
but logical simplifications (such as removal of unneeded parentheses,
or simplification based on evaluating results) are not allowed.
</dl>
<h3 class="heading settled" data-level="7.6" id="the-css-interface"><span class="secno">7.6. </span><span class="content"> The <code>CSS</code> interface, and the <code title="">supports()</code> function</span><a class="self-link" href="#the-css-interface"></a></h3>
<p>The <code class="idl"><a data-link-type="idl" href="https://drafts.csswg.org/cssom-1/#css">CSS</a></code> interface holds useful CSS-related functions that do not belong elsewhere.</p>
<pre class="idl def">partial interface <a class="idl-code" data-link-type="interface" href="https://drafts.csswg.org/cssom-1/#css">CSS</a> {
static boolean <dfn class="idl-code" data-dfn-for="CSS" data-dfn-type="method" data-export="" data-lt="supports(property, value)" id="dom-css-supports">supports<a class="self-link" href="#dom-css-supports"></a></dfn>(DOMString <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/css-conditional-3/#dom-css-supports-property-value-property">property</a>, DOMString <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/css-conditional-3/#dom-css-supports-property-value-value">value</a>);
static boolean <dfn class="idl-code" data-dfn-for="CSS" data-dfn-type="method" data-export="" data-lt="supports(conditionText)" id="dom-css-supports-conditiontext">supports<a class="self-link" href="#dom-css-supports-conditiontext"></a></dfn>(DOMString <a class="idl-code" data-link-type="argument" href="https://drafts.csswg.org/css-conditional-3/#dom-css-supports-conditiontext-conditiontext">conditionText</a>);
};
</pre>
<dl class="idl-methods">
<dt><code>supports(DOMString property, DOMString value)</code>,
returns <code>boolean</code>
<dt><code>supports(DOMString conditionText)</code>,
returns <code>boolean</code>
<dd>
When the <code title="">supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
it must return <code>true</code> if <var>property</var> is a literal match for the name of a CSS property that the UA supports,
and <var>value</var> would be successfully parsed as a supported value for that property.
(Literal match means that no CSS escape processing is performed,
and leading and trailing whitespace are not stripped,
so any leading whitespace, trailing whitespace,
or CSS escapes equivalent to the name of a property
would cause the method to return <code>false</code>.)
Otherwise, it must return <code>false</code>.
<p> When invoked with a single <var>conditionText</var> argument,
it must return <code>true</code> if <var>conditionText</var>,
when parsed and evaluated as a <code>supports_condition</code>,
would return true.
Otherwise, it must return <code>false</code>. </p>
</dl>
<h2 class="heading settled" data-level="8" id="changes"><span class="secno">8. </span><span class="content"> Changes</span><a class="self-link" href="#changes"></a></h2>
<p>The following (non-editorial) changes were made to this specification since the <a href="https://www.w3.org/TR/2012/WD-css3-conditional-20121213/">13 December 2012 Working Draft</a>: </p>
<ul>
<li>Require whitespace around <span class="css">and</span> and <span class="css">or</span> and after <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/mediaqueries-4/#valdef-media-not">not</a>.
<li>Add note explaining that user preferences that effectively disable a property (e.g., high-contrast mode disabling colors) do not effect the definition of support.
<li>Describe requirements for conditionText getter on CSSSupportsRule.
<li>Clarify the definition of “literal match” in CSS.supports().
<li>Specify behavior of CSSGroupingRule.insertRule when given an empty string or more than one syntactically valid rule.
</ul>
<h2 class="no-num heading settled" id="acknowledgments"><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
<p> Thanks to the ideas and feedback from
Tab Atkins,
Arthur Barstow,
Ben Callahan, <span lang="tr">Tantek Çelik</span>,
Alex Danilo,
Elika Etemad,
Pascal Germroth, <span lang="de">Björn Höhrmann</span>,
Paul Irish,
Brad Kemper, <span lang="nl">Anne van Kesteren</span>,
Vitor Menezes,
Alex Mogilevsky,
Chris Moschini,
James Nurthen,
Simon Pieters, <span lang="fr">Florian Rivoal</span>, <span lang="fr">Simon Sapin</span>,
Nicholas Shanks,
Ben Ward,
Zack Weinberg,
Estelle Weyl,
Boris Zbarsky,
and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community. </p>
</main>
<h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
<h3 class="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-conventions"></a></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>
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
<p>Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with <code>class="example"</code>,
like this: </p>
<div class="example" id="example-52448c84">
<a class="self-link" href="#example-52448c84"></a>
<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="note"</code>, like this: </p>
<p class="note" role="note">Note, this is an informative note. </p>
<p>Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <code><strong class="advisement"></code>, like
this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
<h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
<p>Conformance to this specification
is defined for three conformance classes: </p>
<dl>
<dt>style sheet
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
style sheet</a>.
<dt>renderer
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that interprets the semantics of a style sheet and renders
documents that use them.
<dt>authoring tool
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that writes a style sheet.
</dl>
<p>A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module. </p>
<p>A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.) </p>
<p>An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module. </p>
<h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
<p>The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future. </p>
<h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
<p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
(and <a href="http://www.w3.org/TR/CSS2/conform.html#ignore">ignore as appropriate</a>)
any at-rules, properties, property values, keywords, and other syntactic constructs
for which they have no usable level of support</strong>.
In particular, user agents <em>must not</em> selectively ignore
unsupported property values and honor supported values in a single multi-value property declaration:
if any value is considered invalid (as unsupported values must be),
CSS requires that the entire declaration be ignored. </p>
<h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
<p>To avoid clashes with future stable CSS features,
the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
<h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>