design-reviews#1176: Incubation: @supports at-rule

#1176: Incubation: @supports at-rule

Visit on Github

Opened Dec 9, 2025

Explainer

https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/AtRuleFeatureDetection/explainer.md

The explainer

Includes the information requested by the Explainer Explainer.
Follows the Web Platform Design Principles.
Includes or links to answers to the Security/Privacy Questionnaire.
Describes user research you did to validate the problem and/or design.

Where and by whom is the work is being done?

GitHub repo: https://github.com/MicrosoftEdge/MSEdgeExplainers/tree/main/AtRuleFeatureDetection
Primary contacts:
- Kevin Babbit (@kbabbit), Microsoft, Software Engineer
- Diego Gonzalez (@diekus), Microsoft, Program Manager
Organization/project driving the design:
This work is being funded by: Microsoft
Incubation and standards groups that have discussed the design:
- CSSWG F2F August 2025
Standards group(s) that you expect to discuss and/or adopt this work when it's ready: CSSWG

Feedback so far

Multi-stakeholder feedback:
- Chromium comments:
- Mozilla comments: https://github.com/mozilla/standards-positions/issues/1310
- WebKit comments: https://github.com/WebKit/standards-positions/issues/565

You should also know that...

No response

Track conversations at https://tag-github-bot.w3.org/gh/w3ctag/design-reviews/1176

Discussions

Discussed Dec 15, 2025 (See Github)

Christian: Think this is about feature detecting at-rules themselves, kind of meta-feature detection.

Lola: I will have a look at this.

Christian: I will join.

Discussed Jan 5, 2026 (See Github)

CHristian: Had a first look. Seems fine, but want to do another pass. Terminology and other things. This lets you detect @-rule support in the browser. Elaborate.

Yves: Is it an @-rule extension?

Martin: It extends @supports, with a new syntax. My only concern is, it looks fiddly. Documentation is going to important.

Discussed Jan 12, 2026 (See Github)

Christian: No update.

Lola: No update.

Comment by @christianliebel Jan 19, 2026 (See Github)

@diekus @kbabbitt Thank you for your proposal and the great explanation of the problem space. We have a question: The proposal states that at-rule() returns true if the at-rule name is recognized in any context. As you describe, this introduces the risk of contextual false positives. Did you evaluate context-aware detection? If so, why did you prioritize global recognition over contextual validity?

Comment by @kbabbitt Jan 19, 2026 (See Github)

@christianliebel We did consider context-aware detection. When we discussed this problem space at a CSSWG face-to-face meeting last fall, the group resolved to introduce a new @supports-condition as a way of both defining and naming (for reuse) complex support queries such as those that need to account for context, and to keep at-rule() simple as an author convenience. Discussion in https://github.com/w3c/csswg-drafts/issues/12622 and https://github.com/w3c/csswg-drafts/issues/6966#issuecomment-3205037703.