https://drafts.csswg.org/css-contain-3/#container-type
Created attachment 450012 [details] Patch
Created attachment 450022 [details] Patch
Committed r288627 (246444@main): <https://commits.webkit.org/246444@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 450022 [details].
<rdar://problem/88081615>