Firefox 157 Adds at-rule() to @supports and a chain Value for overscroll-behavior

Firefox 157 was released on September 29. The change with the widest reach is at-rule(), a new function usable in @supports and in the supports() condition of @import.

Until now @supports could only ask whether a property or value parses, so there was no way to gate a block on whether the engine understands an at-rule at all. If you have been testing an at-rule's availability by probing some property that shipped alongside it, you can now write @supports at-rule(@scope) and get a direct answer instead.

Adopting it needs no support matrix, which is the useful part: @supports at-rule(...) is itself a feature query, so an engine lacking it fails the query and skips the block. Swap the proxy test for the honest one today and let each engine opt in as it ships support.

The second change is overscroll-behavior: chain. Existing values suppress the browser's boundary behaviour; chain adds a middle option where scrolling passes to the next scrollable ancestor but the default bounce does not fire. That is what you want for a nested scroll area inside a modal or carousel, where the gesture should keep moving without rubber-banding the page behind it.

Two items need no action. WebGPU gained TRANSIENT_ATTACHMENT, relevant only if you write render-pass-only textures. The Animation.reverse() and playbackRate changes are spec-conformance fixes, correcting behaviour rather than adding it.