Spec: https://www.w3.org/TR/css-cascade-5/#cascade-layers Chromium bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1164367
<rdar://problem/73474426>
*** Bug 229289 has been marked as a duplicate of this bug. ***
This was enabled by default in https://trac.webkit.org/changeset/283218/webkit. Further work (spec updates and such) can be handled as separate bugs.