Resync web-platform-tests/custom-elements from upstream.
Created attachment 403660 [details] Patch
Comment on attachment 403660 [details] Patch rs=me, but please fix the red.
Created attachment 403694 [details] Patch
Committed r264020: <https://trac.webkit.org/changeset/264020> All reviewed patches have been landed. Closing bug and clearing flags on attachment 403694 [details].
<rdar://problem/65180416>