at-rule: CSS Feature Detection

Category: CSS
Type: New or changed feature
Status: Proposed (Chrome Proposed)
Intent stage: None

Summary

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

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.

Standards & signals

Specification: https://drafts.csswg.org/css-conditional-5/#support-definition-at-rules
Firefox: No signal
Safari: No signal
Web developers: Positive — There have been several requests for this feature; some examples: https://github.com/w3c/csswg-drafts/issues/9743 https://ruhr.social/@nilsbinder/112721678680237204 https://mastodon.social/@xro/113106213499516093 https://bsky.app/profile/badrihippo.fosstodon.org.ap.brid.gy/post/3m6vupwqhbkk2
Tracking bug: https://issues.chromium.org/issues/40211832

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

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