CSSOM can start inserting/parsing an at-rule already in a nested context (bypass the "normal" nested stack). https://github.com/WebKit/WebKit/blob/main/LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/cssom-expected.txt#L9
rdar://107760234
Pull request: https://github.com/WebKit/WebKit/pull/12810
Committed 263028@main (ab88221743a9): <https://commits.webkit.org/263028@main> Reviewed commits have been landed. Closing PR #12810 and removing active labels.