SSIA I will import this test from Blink / Chromium which uses 'testharness' and will also update as per feedback to remove prefixed CSS and DOM stuff. Thanks!
Committed 271726@main (a75d4f229c6f): <https://commits.webkit.org/271726@main> Reviewed commits have been landed. Closing PR #21481 and removing active labels.
<rdar://problem/119373595>