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<pre class="metadata">
Title: CSS Conditional Rules Module Level 3
Group: csswg
Shortname: css-conditional
Level: 3
Status: CRD
Prepare for TR: yes
Date: 2024-08-15
Work Status: Testing
Implementation Report: https://test.csswg.org/harness/results/css-conditional-3_dev/grouped/
Implementation Report: https://wpt.fyi/results/css/css-conditional?label=master&label=experimental&aligned
WPT Path Prefix: css/css-conditional/
WPT Display: open
ED: https://drafts.csswg.org/css-conditional-3/
TR: https://www.w3.org/TR/css-conditional-3/
Previous Version: https://www.w3.org/TR/2022/CR-css-conditional-3-20220113/
Previous Version: https://www.w3.org/TR/2021/CRD-css-conditional-3-20211223/
Previous Version: https://www.w3.org/TR/2020/CR-css-conditional-3-20201208/
Previous Version: https://www.w3.org/TR/2013/CR-css3-conditional-20130404/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Chris Lilley, W3C, https://svgees.us/, w3cid 1438
Abstract: 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 [[!CSS21]], which builds on CSS level 1
[[CSS1]]. 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.
At Risk: 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.
At Risk: 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.
Default Highlight: css
</pre>
<pre class=link-defaults>
spec:css-color-4; type:property; text:color
spec:html; type:element; text:link
spec:css-namespaces-3; type:dfn; text:namespace prefix
spec:css-namespaces-3; type:dfn; text:CSS qualified name
spec:css-values-4; type:type; text:<general-enclosed>
spec:css-backgrounds-3; type:property; text:box-shadow
spec:css-syntax-3; type:type; text:<declaration>
</pre>
<h2 id="introduction">Introduction</h2>
<h3 id="context">Background</h3>
<em>This section is not normative.</em>
[[!CSS21]] 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
<{link}>. 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.
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.
This specification also defines an additional type of conditional
group rule, ''@supports'', to
address author and user requirements.
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.
<h3 id="placement">Module Interactions</h3>
This module replaces and extends the ''@media'' rule
feature defined in [[!CSS21]] section 7.2.1 and
incorporates the modifications previously made non-normatively by
[[!MEDIAQUERIES-4]] section 1.
<h2 id="processing">Processing of conditional group rules</h2>
This specification defines some CSS [=at-rules=],
called <dfn export lt="conditional group rule">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">
For example, this rule:
<pre>
@media print {
/* hide navigation controls when printing */
#navigation { display: none }
}
</pre>
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>
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.
<wpt title="If condition is true, rules applied in place.">
at-supports-001.html
at-media-001.html
</wpt>
<wpt title="If condition is false, rules not applied.">
css-supports-020.xht
at-media-002.html
</wpt>
<wpt title="CSSOM contains rules regardless of condition.">
js/conditional-CSSGroupingRule.html
</wpt>
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.
<wpt title="Nested rules apply when all conditions are true.">
at-supports-002.html
at-supports-003.html
at-supports-004.html
at-supports-005.html
at-supports-048.html
css-supports-025.xht
css-supports-026.xht
css-supports-046.xht
</wpt>
<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 ''#navigation { display: none }'' applies
whenever this style sheet is applied to print media, and the rule
''.note { float: none }'' is applied only when the style sheet
is applied to print media <em>and</em> the width of the page box is less
than or equal to 12 centimeters.</div>
When the condition for a conditional group rule changes, CSS
processors <strong>must</strong> reflect that the rules now apply or no
longer apply, except for properties whose definitions define effects of
computed values that persist past the lifetime of that value (such as
for some properties in [[CSS3-TRANSITIONS]] and
[[!CSS3-ANIMATIONS]]).
<wpt title="Change in condition simultaneous with change in condition application (except per transition/animation rules).">
at-media-dynamic-001.html
</wpt>
<h2 id="contents-of">
Contents of conditional group rules</h2>
All [=conditional group rules=] are defined to take a <<rule-list>> in their block,
and accept any rule that is normally allowed at the top-level of a stylesheet,
and not otherwise restricted.
(For example, an ''@import'' rule must appear at the actual beginning of a stylesheet,
and so is not valid inside of another rule.)
<wpt title="Valid to nest all unrestricted top-level rules.">
at-supports-content-002.html
at-supports-content-003.html
at-supports-content-004.html
at-media-content-002.html
at-media-content-003.html
at-media-content-004.html
</wpt>
Invalid or unknown rules inside the <<rule-list>> must be considered invalid and ignored,
but do not invalidate the [=conditional group rule=].
<wpt title="Invalid rules do not invalidate conditional group rule.">
at-supports-content-001.html
at-media-content-001.html
</wpt>
Any namespace prefixes used in a [=conditional group rule=]
must have been declared,
otherwise they are invalid.
<div class="example" id=ex-declared-ns>For example, this rule
containing a [=CSS qualified name=] is valid,
because the [=namespace prefix=] has been bound to a namespace url:
<pre>
@namespace x url(http://www.w3.org/1999/xlink);
@supports (content: attr(x|href)) {
// do something }
</pre>
</div>
<div class="example">For example, to determine whether this rule is valid:
<pre>
@supports (content: attr(n|tooltip)) {
// do something }
</pre>
The user agent will consult the namespace map to see whether a namespace url exists corresponding to the "n" prefix.
</div>
<wpt title="Validity of namespace prefixes depends on namespace map.">
at-supports-namespace-001.html
at-supports-namespace-002.html
</wpt>
<h2 id="use">
Placement of conditional group rules</h2>
Conditional group rules are allowed
wherever [=style rules=] are allowed
(at the top-level of a style sheet,
as well as within other conditional group rules).
CSS processors <strong>must</strong> process such rules
as <a href="#processing">described above</a>.
<wpt title="Conditional group rules allowed wherever style rules are allowed.">
at-media-001.html
at-supports-001.html
at-supports-002.html
at-supports-003.html
at-supports-004.html
at-supports-005.html
css-supports-025.xht
css-supports-026.xht
css-supports-046.xht
</wpt>
Any [=at-rules=] that are not allowed after a style rule
(e.g., ''@charset'', ''@import'', or ''@namespace'' rules)
are also not allowed after a conditional group rule,
and are therefore [=invalid=] when so placed.
<wpt title="At rules not allowed after style rule invalid after conditional group rule.">
at-media-003.html
at-supports-045.html
</wpt>
<h2 id="at-media">
Media-specific style sheets: the ''@media'' rule</h2>
The <dfn at-rule id="at-ruledef-media">@media</dfn> rule
is a conditional group rule whose condition is a media query.
Its syntax is:
<pre class="prod def" nohighlight>
@media <<media-query-list>> {
<<rule-list>>
}
</pre>
It consists of the at-keyword ''@media''
followed by a (possibly empty) media query list
(as defined in [[!MEDIAQUERIES-4]]),
followed by a block containing arbitrary rules.
The condition of the rule is the result of the media query.
<div class="example">
This ''@media'' rule:
<pre>
@media screen and (min-width: 35em),
print and (min-width: 40em) {
#section_navigation { float: left; width: 10em; }
}
</pre>
has the condition
''screen and (min-width: 35em), print and (min-width: 40em)'',
which is true for screen displays
whose viewport is at least 35 times the initial font size
and for print displays
whose viewport is at least 40 times the initial font size.
When either of these is true,
the condition of the rule is true,
and the rule
''#section_navigation { float: left; width: 10em; }''
is applied.
</div>
<wpt title="Media rule condition can be empty">
at-media-whitespace-optional-001.html
</wpt>
<wpt title="Media rule condition is media query">
at-media-001.html
at-media-002.html
</wpt>
<wpt title="White space is optional where not required for tokenization.">
at-media-whitespace-optional-001.html
at-media-whitespace-optional-002.html
at-supports-whitespace.html
</wpt>
<h2 id="at-supports">Feature queries: the ''@supports'' rule</h2>
The <dfn at-rule id="at-ruledef-supports">@supports</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.
These queries are called <dfn export>CSS feature queries</dfn> or (colloquially) <dfn export>supports queries</dfn>.
Note: 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.
The syntax of the condition in the ''@supports'' rule
is similar to that defined for <<media-condition>> in [[MEDIAQUERIES-4]],
but without the "unknown" value logic:
* 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.
* conjunction (and) is needed so that
multiple required features can be tested.
* 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.
Therefore, the syntax of the ''@supports'' rule allows
testing for property:value pairs, and arbitrary conjunctions (and),
disjunctions (or), and negations (not) of them.
The syntax of the ''@supports'' rule is:
<pre class="prod def" nohighlight>
@supports <<supports-condition>> {
<<rule-list>>
}
</pre>
with <<supports-condition>> defined as:
<pre class="prod def" nohighlight>
<dfn><supports-condition></dfn> = not <<supports-in-parens>>
| <<supports-in-parens>> [ and <<supports-in-parens>> ]*
| <<supports-in-parens>> [ or <<supports-in-parens>> ]*
<dfn><supports-in-parens></dfn> = ( <<supports-condition>> ) | <<supports-feature>> | <<general-enclosed>>
<dfn><supports-feature></dfn> = <<supports-decl>>
<dfn><supports-decl></dfn> = ( <<declaration>> )
</pre>
The above grammar is purposely very loose for forwards-compatibility reasons,
since the <<general-enclosed>> production
allows for substantial future extensibility.
Any ''@supports'' rule that does not parse according to the grammar above
(that is, a rule that does not match this loose grammar
which includes the <<general-enclosed>> 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).
<wpt title="White space is optional where not required for tokenization.">
css-supports-015.xht
</wpt>
<wpt title="Grammatically-invalid @supports rule is ignored.">
at-supports-019.html
at-supports-020.html
at-supports-021.html
at-supports-022.html
at-supports-023.html
at-supports-024.html
at-supports-025.html
at-supports-026.html
at-supports-027.html
at-supports-028.html
at-supports-029.html
at-supports-030.html
at-supports-031.html
at-supports-032.html
at-supports-033.html
css-supports-034.xht
css-supports-037.xht
</wpt>
Each of these grammar terms is associated with a boolean result,
as follows:
: <<supports-condition>>
: <<supports-in-parens>>
:: The result is the result of the child subexpression.
: not <<supports-in-parens>>
:: The result is the negation of the <<supports-in-parens>> term.
<wpt title="Not negates supports condition.">
at-supports-009.html
at-supports-010.html
css-supports-016.xht
</wpt>
<wpt title="Not must be followed by space.">
at-supports-014.html
css-supports-038.xht
</wpt>
<wpt title="Not requires parentheses.">
css-supports-017.xht
css-supports-018.xht
</wpt>
: <<supports-in-parens>> [ and <<supports-in-parens>> ]*
::
The result is true if all of the <<supports-in-parens>> child terms are true,
and false otherwise.
<wpt title="And condition is true iff all linked conditions are true.">
at-supports-007.html
at-supports-010.html
at-supports-012.html
css-supports-008.xht
css-supports-009.xht
css-supports-010.xht
css-supports-012.xht
</wpt>
<wpt title="And requires parentheses.">
css-supports-019.xht
</wpt>
: <<supports-in-parens>> [ or <<supports-in-parens>> ]*
::
The result is false if all of the <<supports-in-parens>> child terms are false,
and true otherwise.
<wpt title="Or condition is true iff any of linked conditions is true.">
at-supports-008.html
at-supports-010.html
at-supports-013.html
css-supports-006.xht
css-supports-007.xht
css-supports-011.xht
css-supports-021.xht
</wpt>
<wpt title="Or must be followed by space.">
at-supports-043.html
css-supports-039.xht
</wpt>
<wpt title="Or requires parentheses.">
css-supports-029.xht
css-supports-030.xht
</wpt>
: <<supports-decl>>
::
The result is true if the UA [=supports=] the declaration within the parentheses.
<wpt title="Supports condition is true iff declaration is supported.">
at-supports-001.html
at-supports-017.html
at-supports-018.html
css-supports-001.xht
</wpt>
<wpt title="Declaration cannot include semicolon.">
at-supports-038.html
at-supports-039.html
</wpt>
<wpt title="Declaration value can be empty.">
css-supports-022.xht
</wpt>
<wpt title="Declaration cannot include invalid !tokens.">
css-supports-043.xht
css-supports-044.xht
css-supports-045.xht
</wpt>
: <<general-enclosed>>
::
The result is false.
Authors must not use <<general-enclosed>> in their stylesheets.
<span class='note'>It exists only for future-compatibility,
so that new syntax additions do not invalidate too much of a <<supports-condition>> in older user agents.</span>
<wpt title="Unrecognized but grammatically-valid condition is false, not invalid.">
at-supports-015.html
at-supports-046.html
css-supports-023.xht
css-supports-031.xht
css-supports-032.xht
css-supports-033.xht
css-supports-034.xht
css-supports-036.xht
css-supports-040.xht
css-supports-041.xht
css-supports-042.xht
css-supports-046.xht
</wpt>
<wpt title="Brackets/parenthesis must be balanced">
css-supports-035.xht
</wpt>
The condition of the ''@supports'' rule
is the result of the <<supports-condition>> in its prelude.
<div class="example">
For example, the following rule
<pre>
@supports ( display: flex ) {
body, #navigation, #content { display: flex; }
#navigation { background: blue; color: white; }
#article { background: white; color: black; }
}
</pre>
applies the rules inside the ''@supports'' rule only when
''display: flex'' is supported.
</div>
<div class="example">
The following example shows an additional ''@supports'' rule that can
be used to provide an alternative for when ''display: flex'' is not
supported:
<pre>
@supports not ( display: flex ) {
body { width: 100%; height: 100%; background: white; color: black; }
#navigation { width: 25%; }
#article { width: 75%; }
}
</pre>
Note that the 'width' declarations may be harmful to the
flex-based layout, so it is important that they be present only in
the non-flex styles.
</div>
<div class="example">
The following example checks for support for the 'box-shadow'
property, including checking for support for vendor-prefixed versions of
it. When the support is present, it specifies both 'box-shadow' (with
the prefixed versions) and 'border' in a way what would cause the box to
become invisible were 'box-shadow' 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>
To avoid confusion between <css>and</css> and <css>or</css>, the syntax requires
that both <css>and</css> and <css>or</css> 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 <css>and</css>, <css>or</css>,
and <css>not</css> operators to be mixed without a layer of parentheses.
<div class="example">
For example, the following rule is not valid:
<pre class="illegal">
@supports (transition-property: color) or
(animation-name: foo) and
(transform: rotate(10deg)) {
/* ... */
}
</pre>
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>
<wpt title="Parentheses are required to mix operators.">
at-supports-016.html
css-supports-013.xht
css-supports-014.xht
</wpt>
The declaration being tested must always occur within parentheses,
when it is the only thing in the expression.
<div class="example">
For example, the following rule is not valid:
<pre class="illegal">
@supports display: flex {
/* ... */
}
</pre>
Instead, authors must write:
<pre>
@supports (display: flex) {
/* ... */
}
</pre>
</div>
<wpt title="Parentheses are required around declaration test.">
at-supports-011.html
at-supports-034.html
at-supports-035.html
at-supports-036.html
at-supports-037.html
css-supports-002.xht
</wpt>
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">
For example, authors may write:
<pre>
@supports ((display: flex)) {
/* ... */
}
</pre>
</div>
<wpt title="Extra parentheses are allowed.">
at-supports-006.html
css-supports-003.xht
</wpt>
A trailing ''!important'' on a declaration being tested is allowed,
though it won't change the validity of the declaration.
<div class="example">
For example, the following rule is valid:
<pre>
@supports (display: flex !important) {
/* ... */
}
</pre>
</div>
<wpt title="!important is allowed.">
at-supports-007.html
css-supports-004.xht
</wpt>
<h3 id="support-definition">Definition of support</h3>
For forward-compatibility,
<a href="https://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
(Declarations and properties)</a> of [[!CSS21]]
defines rules for handling invalid properties and values.
CSS processors that
do not implement or partially implement a specification
<strong>must</strong> treat any part of a value that they
do not implement, or
do not have a usable level of support for,
as invalid according to this rule
for handling invalid properties and values,
and therefore <strong>must</strong> discard the declaration as a parse error.
A CSS processor is considered to <dfn export for=CSS 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)
within a [=style rule=].
If a processor does not implement, with a usable level of support,
both the property and the value given,
then it <strong>must not</strong>
accept the declaration or claim support for it.
Note: Note that properties or values
whose support is effectively disabled by user preferences
are still considered as supported by this definition.
For example, if a user has enabled a high-contrast mode
that causes colors to be overridden,
the CSS processor is still considered to support the 'color' property
even though declarations of the 'color' property may have no effect.
On the other hand, a developer-facing preference
whose purpose is to enable or disable support for an experimental CSS feature
does affect this definition of support.
These rules (and the equivalence between them) allow
authors to use fallback (either in the [[CSS1]] sense of declarations
that are overridden by later declarations or with the new capabilities
provided by the ''@supports'' rule in this specification) that works
correctly for the features implemented. This applies especially to
compound values; implementations must implement all parts of the value
in order to consider the declaration supported, either inside a style rule
or in the declaration condition of an ''@supports'' rule.
<wpt title="Supports queries are true iff property declaration (including all values) is parsed/supported.">
css-supports-005.xht
css-supports-020.xht
css-supports-024.xht
js/CSS-supports-CSSStyleDeclaration.html
at-supports-044.html
</wpt>
<h2 id="apis">APIs</h2>
<wpt>
idlharness.html
js/001.html
</wpt>
<h3 id='extensions-to-cssrule-interface'>
Extensions to the <code>CSSRule</code> interface</h3>
The <code>CSSRule</code> interface is extended as follows:
<pre class='idl'>
partial interface CSSRule {
const unsigned short SUPPORTS_RULE = 12;
<!--
const unsigned short DOCUMENT_RULE = 13;
-->
};
</pre>
<wpt title="CSSRule.SUPPORTS_RULE = 12">
js/conditional-CSSGroupingRule.html
</wpt>
<h3 id="the-cssconditionrule-interface">
The <code>CSSConditionRule</code> interface</h3>
The {{CSSConditionRule}} interface represents
all the “conditional” at-rules,
which consist of a condition and a statement block.
<pre class='idl' export>
[Exposed=Window]
interface CSSConditionRule : CSSGroupingRule {
readonly attribute CSSOMString conditionText;
};
</pre>
<wpt title="CSSConditionRule inherits from CSSGroupingRule.">
js/conditional-CSSGroupingRule.html
</wpt>
<wpt title="CSSConditionRule has .conditionText attribute."></wpt>
<dl class='idl-attributes'>
<dt><code>conditionText</code> of type <code>CSSOMString</code>
<dd>
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:
The <code>conditionText</code> attribute, on getting, must return
the result of serializing the associated condition.
<wpt title=".conditionText returns serialization of condition."></wpt>
</dl>
<h3 id="the-cssmediarule-interface">
The <code>CSSMediaRule</code> interface</h3>
The {{CSSMediaRule}} interface represents a ''@media'' at-rule:
<pre class='idl'>
[Exposed=Window]
interface CSSMediaRule : CSSConditionRule {
[SameObject, PutForwards=mediaText] readonly attribute MediaList media;
readonly attribute boolean matches;
};
</pre>
<!-- <wpt title="CSSMediaRule inherits from CSSConditionRule.">
js/CSSConditionRule.html
</wpt>
<wpt title="CSSMediaRule represents an @media rule."></wpt> -->
<dl class='idl-attributes'>
<dt><code>media</code> of type {{MediaList}}, readonly
<dd>The <code>media</code> attribute must return a {{MediaList}} object
for the list of media queries specified with the ''@media'' at-rule.
<wpt title=".media returns a MediaList matching the @media condition."></wpt>
<dt><code>matches</code> of type {{boolean}}, readonly
<dd>The <code>matches</code> attribute returns true
if the rule is in an stylesheet attached to a document
whose {{Window}} matches this rule’s {{CSSMediaRule/media}} [=media query=],
and returns false otherwise.
<wpt title=".matches matches the media query, returns boolean."></wpt>
<dt><code>conditionText</code> of type <code>CSSOMString</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.
<wpt title="Value of CSSMediaRule.conditionText matches value of media.mediaText."></wpt>
</dl>
<h3 id="the-csssupportsrule-interface">
The <code>CSSSupportsRule</code> interface</h3>
The {{CSSSupportsRule}} interface represents a ''@supports'' rule.
<pre class='idl'>
[Exposed=Window]
interface CSSSupportsRule : CSSConditionRule {
readonly attribute boolean matches;
};
</pre>
<!-- <wpt title="CSSSupportsRule inherits from CSSConditionRule.">
js/CSSConditionRule.html
</wpt>
<wpt title="CSSSupportsRule represents an @supports rule."></wpt> -->
<dl class='idl-attributes'>
<dt><code>matches</code> of type {{boolean}}, readonly
<dd>The <code>matches</code> attribute returns the evaluation of
the [=CSS feature query=] represented in {{CSSConditionRule/conditionText}}.
<wpt title="CSSSupportsRule.matches returns true if matches feature query"></wpt>
<dt><code>conditionText</code> of type <code>CSSOMString</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 <<general-enclosed>> extensibility 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.
<wpt title="CSSSupportsRule.conditionText can have tokenization simplifications."></wpt>
<wpt title="CSSSupportsRule.conditionText cannot have other simplifications."></wpt>
</dl>
<!--
<h3 id="the-cssdocumentrule-interface">
The <code>CSSDocumentRule</code> interface</h3>
The {{CSSDocumentRule}} interface represents a ''@document'' rule.
<pre class='idl'>
[Exposed=Window]
interface CSSDocumentRule : CSSConditionRule {
};
</pre>
-->
<h3 id='the-css-namespace'>
<span id='the-css-interface'>The <code>CSS</code> namespace, and the <code title=''>supports()</code> function</span></h3>
The {{CSS}} namespace holds useful CSS-related functions that do not belong elsewhere.
<pre class='idl'>
partial namespace CSS {
boolean supports(CSSOMString property, CSSOMString value);
boolean supports(CSSOMString conditionText);
};
</pre>
<dl class='idl-methods'>
<dt><code>supports(CSSOMString property, CSSOMString value)</code>, returns <code>boolean</code>
<dd>
<div algorithm="supports(property, value)">
When the {{supports(property, value)}} method is invoked
with two arguments <var>property</var> and <var>value</var>:
1. If |property| is an [=ASCII case-insensitive=] match
for any defined CSS property that the UA supports,
or is a [=custom property name string=],
and |value| successfully [=CSS/parses=] according to that property's grammar,
return <code>true</code>.
2. Otherwise, return <code>false</code>.
Note: No CSS escape or whitespace processing is performed on the property name,
so <code>CSS.supports(" width", "5px")</code> will return <code>false</code>,
as " width" isn't the name of any property due to the leading space.
Note: ''!important'' flags are not part of property grammars,
and will cause |value| to parse as invalid,
just as they would in the value argument to ''element.style.setProperty()''.
</div>
<wpt title="CSS.supports(arg1, arg2) evaluates support of property arg1 with value arg2."></wpt>
<dt><code>supports(CSSOMString conditionText)</code>, returns <code>boolean</code>
<dd>
<div algorithm="supports(conditionText)">
When the {{supports(conditionText)}} method is invoked
with a single <var>conditionText</var> argument:
1. If |conditionText|,
[=CSS/parsed=] and evaluated as a <<supports-condition>>,
would return true,
return <code>true</code>.
2. Otherwise,
If |conditionText|,
wrapped in parentheses
and then [=CSS/parsed=] and evaluated as a <<supports-condition>>,
would return true,
return <code>true</code>.
3. Otherwise, return <code>false</code>.
All namespaces in the <var>conditionText</var> argument
are considered invalid,
just as they are in <code>document.querySelector("a|b")</code>.
</div>
<wpt title="CSS.supports(arg1) evaluates supports condition arg1."></wpt>
<wpt title="CSS.supports(arg1) implies parentheses.">
js/supports-conditionText.html
</wpt>
</dl>
<wpt>
js/CSS-supports-L3.html
</wpt>
<h2 class=no-num id=security>Security Considerations</h2>
This spec introduces no new security considerations.
<h2 class=no-num id=privacy>Privacy Considerations</h2>
Various features in this specification,
associated mainly with the ''@media'' rule
but also to some degree with the ''@supports'' rule,
provide information to Web content about
the user's hardware and software and their configuration and state.
Most of the information is provided through the features in [[MEDIAQUERIES-4]]
rather than through the features in this specification.
However, the ''@supports'' rule may provide some additional details about the user's software
and whether it is running with non-default settings that may enable or disable certain features.
Most of this information can also be determined through other APIs.
However, the features in this specification are one of the ways this information
is exposed on the Web.
This information can also, in aggregate, be used to improve the accuracy of
<a href="https://www.w3.org/2001/tag/doc/unsanctioned-tracking/">fingerprinting</a> of the user.
<h2 id="changes">
Changes</h2>
The following (non-editorial) changes were made to this specification since the
<a href="https://www.w3.org/TR/2022/CR-css-conditional-3-20220113/">13 January 2022 Candidate Recommendation Snapshot</a>:
<ul id="changes-20220113">
<!-- to 12 Aug 2024 -->
<li>Clarified that supports() must return false for invalid custom property values</li>
<li>Fixed Web IDL, "bool" should have been "boolean"</li>
<!-- to 24 June 2024 -->
<li>Clarified that a processor must support both the property and the value (<a href="https://github.com/w3c/csswg-drafts/issues/8795">Issue 8795</a>)</li>
<li>Added .matches to @media and @supports (<a href="https://github.com/w3c/csswg-drafts/issues/4240">Issue 4240</a>)</li>
<li>Updated to the new parsing algo names and block production names.</li>
<li>Removed procedure to set readonly CSSMediaRule.conditionText (<a href="https://github.com/w3c/csswg-drafts/pull/8796">PR 8796</a>)</li>
<li>Made conditionText readonly.</li>
</ul>