[css-view-transition-2] `view-transition-class` and tree-scoping

[noamr]

Following up on #10145, we should determine whether elements can define a view-transition-class inside shadow DOM that gets matched to a corresponding class in the pseudo-element (e.g. if the element is a ::part)..
Based on https://drafts.csswg.org/css-scoping-1/#shadow-names, I think it makes sense to allow that, as a view-transition-class is similar to giving an element a font-family that's defined in the root inside a @font-face.

[noamr]

Thinking about this again, view-transition-class is not exactly a name-reference, but rather a name by itself, and the reference is in the pseudo-elements. It's more equivalent to container queries where the query has to be below the definition of the name.

I think the right approach here would be that view-transition-class is a scoped name, and is read from the same scope as the view-transition-name.

The only "quirk" about this is that the shadow DOM can define view-transition-class: none !important or some such and make a ::part not receive a view-transition-class. Perhaps that's OK.

[noamr]

@nt1m @vmpstr @fantasai @khushalsagar any objections here? I think this is a non-controversial fix.

[nt1m]

I guess it would be consistent with the name, which is fine.

[noamr]

@astearns ok to do an offline resolution on this one?

[astearns]

@noamr more comments and/or thumbs-ups would be good for an async resolution. Add a proposed resolution and the Async Resolution: Proposed tag if you’d like to go that route.

[noamr]

Proposed resolution: view-transition-class is a tree-scoped name. It has to match the tree-scope of the view transition when read. De-facto, this means that it only applies in document-scoped stylesheets, and shadow-DOM can reset it to none using !important.

[khushalsagar]

SGTM. So both view-transition-name and view-transition-class have to match the tree-scope of the view transition.

[astearns]

The CSSWG will automatically accept this resolution one week from now if no objections are raised here. Anyone can add an emoji to this comment to express support. If you do not support this resolution, please add a new comment.

Proposed resolution: view-transition-class is a tree-scoped name.