See test case.
Created attachment 10512 [details] Test Case
Related to #28867 - fieldset legends are magically styled.
Safari TP passes the tests in https://github.com/web-platform-tests/wpt/pull/12525 https://github.com/web-platform-tests/wpt/pull/12522 so I think this bug can be closed.
https://github.com/web-platform-tests/wpt/commit/0a87465c2a1eb274e6e045d7ef4d0bb016aa25dc unfortunately added more to that test, http://wpt.live/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-containing-block.html and we fail.
<rdar://problem/95637254>