Even though a custom element is cached in one world, it should return an unknown HTMl element in a different world. <rdar://problem/57881013>
Created attachment 391707 [details] proposed patch.
The ios-wk2 bot failures are unrelated. Landing the patch now.
Comment on attachment 391707 [details] proposed patch. Clearing flags on attachment: 391707 Committed r257414: <https://trac.webkit.org/changeset/257414>
All reviewed patches have been landed. Closing bug.