See https://bugs.chromium.org/p/chromium/issues/detail?id=1138849 / https://github.com/w3c/csswg-drafts/issues/4282. Gecko already ships this.
Created attachment 411698 [details] Patch
Created attachment 411704 [details] Patch
Comment on attachment 411704 [details] Patch Excellent. Thanks Emilio.
Comment on attachment 411704 [details] Patch Thanks for the review Sam!
Committed r268659: <https://trac.webkit.org/changeset/268659> All reviewed patches have been landed. Closing bug and clearing flags on attachment 411704 [details].
<rdar://problem/70418592>