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.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This 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 technical reports index at https://www.w3.org/TR/.
This document was published
by the CSS Working Group as a Working Draft using the Recommendation
track.
Publication as a Working Draft
does not imply endorsement by W3C and its Members.
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 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 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 which the individual believes
contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
1. Introduction
The 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.
The result is true if the UA supports the selector provided as an argument to the function.
2.1. Extensions to the definition of support
A CSS processor is considered to support a CSS selector if it accepts that selector
(rather than discarding it as a parse error),
and that selector doesn’t contain unknown -webkit- pseudo-elements.
Security Considerations
No Security issues have been raised against this document
Privacy Considerations
The 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.
Acknowledgments
The editors would like to thank
all of the contributors to the previous level of this module.
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.
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.
Conformance classes
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 implementations
So 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 Features
Once 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.