This module contains the features of CSS for conditional processing of parts of style sheets, based on capabilities of the processor or the environment the style sheet is being applied in. It includes and extends the functionality of CSS Conditional 3 [css-conditional-3], adding the ability to query support for particular selectors [SELECTORS-4] through the new selector() notation for supports queries.
CSSis a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.
Status of this documentThis section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C standards and drafts index.
This document was published by the CSS Working Group as a Candidate Recommendation Draft using the Recommendation track. Publication as a Candidate Recommendation does not imply endorsement by W3C and its Members. A Candidate Recommendation Draft integrates changes from the previous Candidate Recommendation that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than a work in progress.
Please send feedback by filing issues in GitHub (preferred), including the spec code “css-conditional” in the title, like this: “[css-conditional] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.
This document is governed by the 18 August 2025 W3C Process Document.
This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures 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 that the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
Table of ContentsThe features in level 3 are still defined in [css-conditional-3] and have not yet been copied here.
This level adds extensions to the @supports rule to allow testing for supported selectors.
2. Extensions to the @supports ruleThis level of the specification extends the <supports-feature> syntax as follows:
<supports-feature> = <supports-selector-fn> | <supports-decl> <supports-selector-fn> = selector( <complex-selector> )
The result is true if the UA supports the selector provided as an argument to the function.
This example tests whether the
column combinator(||) is supported in selectors, and if so uses it to style particular cells in a table.
@supports selector(col || td) {
col.selected || td {
background: tan;
}
}
Any namespace prefixes used in a conditional group rule must have been declared, otherwise they are invalid [css-conditional-3]. This includes namespace prefixes inside the selector function.
TestsThis example tries to check that attribute selectors with
CSS qualified namesare supported, but is invalid, because the namespace prefix has not been declared.
@supports selector(a[xlink|href]) {
// do something, but fail
}
This example checks that attribute selectors with
CSS qualified namesare supported.
@namespace x url(http://www.w3.org/1999/xlink);
@supports selector(a[x|href]) {
// do something
}
2.1. Extensions to the definition of support
A CSS processor is considered to support a CSS selector if it accepts that all aspects of that selector, recursively, (rather than considering any of its syntax to be unknown or invalid) and that selector doesn’t contain unknown -webkit- pseudo-elements.
Note: Some functional selectors are parsed forgivingly, i.e. if some arguments are unknown/invalid, the selector itself is not invalidated. These are nonetheless unsupported
Security ConsiderationsNo Security issues have been raised against this document
Privacy ConsiderationsThe selector() function may provide information about the user’s software such as its version and whether it is running with non-default settings that enable or disable certain features.
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 fingerprinting of the user.
AcknowledgmentsThe editors would like to thank all of the contributors to the previous level of this module.
Changes Changes since the Candidate Recommendation Snapshot of 17 February 2022Conformance 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.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class= "example" , like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the normative text with class= "note" , like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class= "advisement" >, like this: UAs MUST provide an accessible alternative.
Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.
Conformance to this specification is defined for three conformance classes:
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.
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.)
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.
Partial implementationsSo that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component 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.
Implementations of Unstable and Proprietary FeaturesTo avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.
Non-experimental implementationsOnce a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
CR exit criteriaFor this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
Index Terms defined by this specificationThe features in level 3 are still defined in
[css-conditional-3]and have not yet been copied here.
↵RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4