Currently CSSValueList supports appending and prepending values. Instead we will make it immutable, as a prelude to making it use less memory.
<rdar://problem/105342727>
Pull request: https://github.com/WebKit/WebKit/pull/10254
Committed 260566@main (bae83919e283): <https://commits.webkit.org/260566@main> Reviewed commits have been landed. Closing PR #10254 and removing active labels.