2060754 - Implement @support at-rule() behind pref

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[Sajid Anwar [:sajidanwar]]

Creating this as the implementation bug for Bug 1751188. This support condition will be implemented behind the layout.css.supports.at-rule.enabled pref, enabled in Nightly by default.

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Release Note Request (optional, but appreciated)
[Why is this notable]: New CSS feature, parity with Chrome
[Affects Firefox for Android]: Yes
[Suggested wording]: Firefox Nightly now supports the at-rule() function for @supports. This allows authors to detect for browser support of specific at-rules, such as @supports at-rule(@scope).
[Links (documentation, blog post, etc)]:

• https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@supports#at-rule
• https://www.bram.us/2026/03/15/at-rule/

Comment 1

[Sajid Anwar [:sajidanwar]]

Attachment: Bug 2060754 - Implement at-rule() function for CSS @supports. r=#style

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/39c90c25677e
https://hg.mozilla.org/integration/autoland/rev/f667e9c47508
Implement at-rule() function for CSS @supports. r=firefox-style-system-reviewers,emilio

Comment 3

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/61745 for changes under testing/web-platform/tests

Comment 4

[pstanciu]

https://hg.mozilla.org/mozilla-central/rev/f667e9c47508

Comment 5

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot

Comment 6

[Ryan VanderMeulen [:RyanVM]]

Added to the Nightly relnotes.

Comment 7

[Dianna Smith [:diannaS]]

removing from nightly-only relnote flag since this is riding the trains with 157 in bug 2060755