This is just converting a CSSValue to OptionSet<Containment>, we don't need custom appliers.
<rdar://problem/112329429>
Pull request: https://github.com/WebKit/WebKit/pull/15868
Committed 266087@main (6e398514a59d): <https://commits.webkit.org/266087@main> Reviewed commits have been landed. Closing PR #15868 and removing active labels.