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For elements whose layout is governed by the CSS box model, any value other than none for the transform property also causes the element to establish a containing block for all descendants. https://www.w3.org/TR/css-transforms-1/
Created attachment 359376 [details] Patch
You could update the comment above too.
Committed r240111: <https://trac.webkit.org/changeset/240111>
<rdar://problem/47351671>