Intent to Ship: at-rule: CSS Feature Detection

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Kevin Babbitt

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Mar 11, 2026, 5:19:01 PM (3 days ago) Mar 11
to blink-dev
Note: This feature is a bit unusual in that it was implemented behind a flag but not advertised on blink-dev with an I2P or dev trial announcement. I picked it up after initial implementation, got clarity on some open spec issues, and adjusted the implementation to match CSSWG resolutions.

Contact emails
Explainer
Specification
Summary
This feature adds an `at-rule()` function to CSS `@supports` which enables authors to feature-detect support for CSS at-rules.

Blink component
Web Feature ID
Motivation
Feature detection is a W3C TAG design principle and a tool that Web authors rely on for graceful degradation of their pages. Currently, the only way for web developers to detect support for CSS at-rules is to use JavaScript. This feature allows at-rule support to be detected using the same `@supports` feature used to detect support for CSS properties.

Initial public proposal
TAG review
TAG review status
Issues addressed

Goals for experimentation
None

Risks


Interoperability and Compatibility
WebView application risks
Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
N/A


Debuggability
Exposure in Dev Tools is the same as for existing `@supports` rules.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?
Yes

Is this feature fully tested by web-platform-tests?
Flag name on about://flags
enable-experimental-web-platform-features

Finch feature name
CSSSupportsAtRuleFunction

Rollout plan
Will ship enabled for all users

Requires code in //chrome?
False

Tracking bug
Estimated milestones
Shipping on desktop
148
DevTrial on desktop
143
Shipping on Android
148
DevTrial on Android
143
Shipping on WebView
148


Anticipated spec changes
Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).
No known open issues on `@supports at-rule()` itself. There were some expansions to the feature discussed to test for certain descriptors or context, but the CSSWG resolved to separate these more detailed queries out to a separate feature `@supports-condition`; see discussion at https://github.com/w3c/csswg-drafts/issues/12622.

Link to entry on the Chrome Platform Status
This intent message was generated by Chrome Platform Status.

Philip Jägenstedt

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Mar 12, 2026, 6:33:32 PM (2 days ago) Mar 12
to Kevin Babbitt, blink-dev
LGTM1, this is a clear gap in feature detection, thanks for closing it!

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Mike Taylor

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Mar 13, 2026, 9:21:58 AM (24 hours ago) Mar 13
to Philip Jägenstedt, Kevin Babbitt, blink-dev

LGTM2 (and thank you for the great explainer).

Chris Harrelson

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Mar 13, 2026, 10:47:47 AM (23 hours ago) Mar 13
to Mike Taylor, Philip Jägenstedt, Kevin Babbitt, blink-dev
LGTM3

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