Implement parsing support for nested style rule ("option 3") https://drafts.csswg.org/css-syntax/#parse-style-blocks-contents Part of the more general css-nesting-1 bug 223497
<rdar://103146832>
Pull request: https://github.com/WebKit/WebKit/pull/7610
Committed 258079@main (2dc309f4a725): <https://commits.webkit.org/258079@main> Reviewed commits have been landed. Closing PR #7610 and removing active labels.