Implement support for StylePropertyMap.append(): - https://drafts.css-houdini.org/css-typed-om/#dom-stylepropertymap-append
Pull request: https://github.com/WebKit/WebKit/pull/6866
Committed 257123@main (30535abeb949): <https://commits.webkit.org/257123@main> Reviewed commits have been landed. Closing PR #6866 and removing active labels.
<rdar://problem/102770418>