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Adds the at-rule feature #3449

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Adds the at-rule feature #3449

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6893073
Adds the at-rule feature.
diekus Oct 14, 2025
09ac973
Create at-rule.yml
diekus Oct 14, 2025
fd00209
Merge branch 'main' into diekus-atRuleFeature
captainbrosset Oct 14, 2025
1d7e54a
Update features/at-rule.yml
diekus Oct 14, 2025
4d00414
Update features/at-rule.yml
diekus Oct 14, 2025
e749fbd
Changes ID to supports-at-rule
diekus Oct 17, 2025
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7 changes: 7 additions & 0 deletions features/supports-at-rule.yml
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Original file line number Diff line number Diff line change
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name: supports-at-rule()
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Sorry, I didn't explain myself here very well. The name before was fine with me, it was only the ID/filename that needed to change.

Suggested change
name: supports-at-rule()
name: at-rule()

description: "The `at-rule()` function, when used with `@supports`, checks if a CSS at-rule is supported. For example `@supports at-rule(@starting-style)` checks if the `@starting-style` at-rule is supported."
spec:
https://www.w3.org/TR/css-conditional-5/
https://github.com/w3c/csswg-drafts/pull/12945
https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/AtRuleFeatureDetection/explainer.md
Comment on lines +4 to +6
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I'm the one who asked Diego to add all 3 URLs here, but we should choose one. @ddbeck do you think it's better to link to the spec (even if doesn't have the changes for this feature in it yet), the PR that's making the changes, or the explainer?

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Looking at other features, it seems like we could choose between pointing to the explainer or the PR. I would vote for the PR because this way it's easy to later check if the PR has been merged, and update the link to the proper spec.
If we link to the explainer, it's maybe more useful for developers, but there's no easy sign to know that an explainer has graduated to a spec, and which one.

So let's perhaps use the PR URL only.

But, also, the build will fail if we do this. To make the build happy, please also add the PR URL to the defaultAllowlist array in /scripts/specs.ts, with a reason why the PR is allowed (e.g. because that's the closest thing we have to a spec).

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Of the 3 links suggested, I think we should only link the w3c/csswg-drafts#12945. That's something that a script can look at and determine when the PR is merged and the spec URL can be updated.

@ddbeck has proposed that we should use proposal instead of spec for this situation, and that would avoid the defaultAllowlist problem.

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@ddbeck ddbeck Oct 16, 2025 •
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Yes, I like the idea of linking to, in order of preference:

  1. An actual spec known to browser-specs
  2. A PR against an actual spec known to browser-specs
  3. Any other PR
  4. Any other URL

#2647 exists to track this idea though it doesn't mention the proposal behavior. I'll put a comment there file a new issue. (edit: no it doesn't, wrong thing)

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I created #3470

group: css
6 changes: 6 additions & 0 deletions features/supports-at-rule.yml.dist
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# Generated from: supports-at-rule.yml
# Do not edit this file by hand. Edit the source file instead!

status:
baseline: false
support: {}