The WebIDL change is in https://github.com/whatwg/webidl/pull/1098. This is a pre-requisite for https://github.com/whatwg/fetch/pull/1392.
<rdar://problem/92540704>
Pull request: https://github.com/WebKit/WebKit/pull/10087
Committed 261472@main (030b6006721b): <https://commits.webkit.org/261472@main> Reviewed commits have been landed. Closing PR #10087 and removing active labels.