borderless-attachment should be restricted behind useSystemAppearance. Although I wonder if we could make this work through appearance: auto + borderless attribute, so we can stop exposing the CSS value altogether, though that would require consumers of <attachment> + appearance: borderless-attachment to change to <attachment borderless>. attachment is implemented via appearance: auto, so we shouldn't need to keep the CSS value around.
<rdar://problem/93331974>
> behind useSystemAppearance Please use attachmentElementEnabled instead.
Pull Request: https://github.com/WebKit/WebKit/pull/649
Committed r294267 (250615@main): <https://commits.webkit.org/250615@main> Reviewed commits have been landed. Closing PR #649 and removing active labels.