CSS Conditional Rules Module Level 4

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.

2. Extensions to the @supports rule

This level of the specification extends the <supports-feature> syntax as follows:

<supports-feature> = <supports-selector-fn> | <supports-decl>
<supports-selector-fn> = selector( <complex-selector> )

<supports-selector-fn>

The result is true if the UA supports the selector provided as an argument to the function.

@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.

@supports selector(a[xlink|href]) {
  // do something, but fail
  }
}

@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 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.

Changes

Changes since the Candidate Recommendation Snapshot of 17 February 2022

• Clarify that unknown or invalid portions of a selector that do not invalidate the selector nonetheless cause the selector to be considered unsupported.

Changes since the First Public Working Draft of 3 March 2020

• Added Privacy and Security sections.
• Added some examples
• Clarified that the requirement to declare namespace prefixes applies to selectors inside selector() (Issue 3220)

Additions since Level 3

• Added selector() notation to supports queries.