As per https://github.com/WebKit/WebKit/pull/6424#issuecomment-1312935702 we could move some code around and reestablish the assertion that got removed. I wasn't able to find a test we would currently fail: https://github.com/web-platform-tests/wpt/pull/38819. Relevant specification change: https://github.com/whatwg/html/pull/8943.
<rdar://problem/106257269>
*** This bug has been marked as a duplicate of bug 252893 ***