web-platform-tests/wpt merge_pr_47948

on GitHub

Part 2: Support :only-child for view transition selectors.

Introduce the concept of pseudo element tree and make sure we achieve
the following behaviors:

1. ::view-transition doesn't accept any non-functional pseudo class
   after it.
2. ::view-transition-*(name) accepts only only-child pseudo class
   after it.

So for named view transition pseudo elements, we make them accept
non-functional pseudo classes and tree structural pseudo classes after
them, for consistency. However, only :only-child is allowed.

Differential Revision: https://phabricator.services.mozilla.com/D219714

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1911216
gecko-commit: 2a950d7be993124346592385bb82f3d3207fe9bf
gecko-reviewers: layout-reviewers, firefox-style-system-reviewers, emilio