WebKit implements this attribute now but doesn't expose it in the list of attribute names (on macOS)
<rdar://problem/125878207>
Created attachment 470752 [details] Patch
Created attachment 470764 [details] Patch
Committed 277117@main (f26be3ca90d5): <https://commits.webkit.org/277117@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 470764 [details].