The CSS Containment specification was implemented in Bug 172026. Now that it has been implemented, we should enable the feature.
<rdar://problem/86218835>
Created attachment 446634 [details] Patch
Committed r286828 (245061@main): <https://commits.webkit.org/245061@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 446634 [details].