anchor.relList.supports("opener") should return true since we support "opener" on anchor elements.
Created attachment 442209 [details] Patch
Created attachment 442223 [details] Patch
Committed r284745 (243454@main): <https://commits.webkit.org/243454@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 442223 [details].
<rdar://problem/84580626>