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>

<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.

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.

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.

For example, this rule:

@media print {

/* hide navigation controls when printing */

#navigation { display: none }

}

causes a particular CSS rule

(making elements with ID &ldquo;navigation&rdquo; be display:none)

apply only when the style sheet is used for a print medium.

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.

at-supports-001.html

at-media-001.html

css-supports-020.xht

at-media-002.html

js/conditional-CSSGroupingRule.html

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.

at-supports-002.html

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<div class="example">For example, with this set of nested rules:

@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 }

}

}

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]]).

at-media-dynamic-001.html

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.)

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

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Invalid or unknown rules inside the <<rule-list>> must be considered invalid and ignored,

but do not invalidate the [=conditional group rule=].

at-supports-content-001.html

at-media-content-001.html

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 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.

at-supports-namespace-001.html

at-supports-namespace-002.html

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>.

at-media-001.html

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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.

at-media-003.html

at-supports-045.html

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:

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.

This ''@media'' rule:

@media screen and (min-width: 35em),

print and (min-width: 40em) {

#section_navigation { float: left; width: 10em; }

}

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

is applied.

at-media-whitespace-optional-001.html

at-media-001.html

at-media-002.html

at-media-whitespace-optional-001.html

at-media-whitespace-optional-002.html

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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:

with <<supports-condition>> defined as:

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).

css-supports-015.xht

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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

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css-supports-016.xht

</wpt>

<wpt title="Not must be followed by space.">

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</wpt>

<wpt title="Not requires parentheses.">

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</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

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</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.">

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</wpt>

<wpt title="Or must be followed by space.">

at-supports-043.html

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</wpt>

<wpt title="Or requires parentheses.">

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</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

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</wpt>

<wpt title="Declaration cannot include semicolon.">

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</wpt>

<wpt title="Declaration value can be empty.">

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</wpt>

<wpt title="Declaration cannot include invalid !tokens.">

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</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.">

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</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.

For example, the following rule

@supports ( display: flex ) {

body, #navigation, #content { display: flex; }

#navigation { background: blue; color: white; }

#article { background: white; color: black; }

}

applies the rules inside the ''@supports'' rule only when

''display: flex'' is supported.

The following example shows an additional ''@supports'' rule that can

be used to provide an alternative for when ''display: flex'' is not

supported:

@supports not ( display: flex ) {

body { width: 100%; height: 100%; background: white; color: black; }

#navigation { width: 25%; }

#article { width: 75%; }

}

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.

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.

.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;

}

}

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.

For example, the following rule is not valid:

@supports (transition-property: color) or

(animation-name: foo) and

(transform: rotate(10deg)) {

/* ... */

}

Instead, authors must write one of the following:

@supports ((transition-property: color) or

(animation-name: foo)) and

(transform: rotate(10deg)) {

/* ... */

}

@supports (transition-property: color) or

((animation-name: foo) and

(transform: rotate(10deg))) {

/* ... */

}

at-supports-016.html

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The declaration being tested must always occur within parentheses,

when it is the only thing in the expression.

For example, the following rule is not valid:

@supports display: flex {

/* ... */

}

Instead, authors must write:

@supports (display: flex) {

/* ... */

}

at-supports-011.html

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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.

For example, authors may write:

@supports ((display: flex)) {

/* ... */

}

at-supports-006.html

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A trailing ''!important'' on a declaration being tested is allowed,

though it won't change the validity of the declaration.

For example, the following rule is valid:

@supports (display: flex !important) {

/* ... */

}

at-supports-007.html

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For forward-compatibility,

(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.

css-supports-005.xht

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js/CSS-supports-CSSStyleDeclaration.html

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idlharness.html

js/001.html

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

partial interface CSSRule {

const unsigned short SUPPORTS_RULE = 12;

<!--

const unsigned short DOCUMENT_RULE = 13;

-->

};

js/conditional-CSSGroupingRule.html

The {{CSSConditionRule}} interface represents

all the &ldquo;conditional&rdquo; at-rules,

which consist of a condition and a statement block.

interface CSSConditionRule : CSSGroupingRule {

readonly attribute CSSOMString conditionText;

};

js/conditional-CSSGroupingRule.html

<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>

The {{CSSMediaRule}} interface represents a ''@media'' at-rule:

interface CSSMediaRule : CSSConditionRule {

[SameObject, PutForwards=mediaText] readonly attribute MediaList media;

readonly attribute boolean matches;

};

<!-- <wpt title="CSSMediaRule inherits from CSSConditionRule.">

js/CSSConditionRule.html

<wpt title="CSSMediaRule represents an @media rule."></wpt> -->

<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>

The {{CSSSupportsRule}} interface represents a ''@supports'' rule.

interface CSSSupportsRule : CSSConditionRule {

readonly attribute boolean matches;

};

<!-- <wpt title="CSSSupportsRule inherits from CSSConditionRule.">

js/CSSConditionRule.html

<wpt title="CSSSupportsRule represents an @supports rule."></wpt> -->

<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>

<!--

The {{CSSDocumentRule}} interface represents a ''@document'' rule.

interface CSSDocumentRule : CSSConditionRule {

};

-->

The {{CSS}} namespace holds useful CSS-related functions that do not belong elsewhere.

partial namespace CSS {

boolean supports(CSSOMString property, CSSOMString value);

boolean supports(CSSOMString conditionText);

};

<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>

js/CSS-supports-L3.html

This spec introduces no new security considerations.

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.

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>:

<!-- 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>

The following (non-editorial) changes were made to this specification since the