Some appearance property values are only used by Apple/WkWebView apps.
<rdar://problem/93254798>
At this point, it's pretty much just `default-button`.
Pull request: https://github.com/WebKit/WebKit/pull/884
Committed r294647 (250871@main): <https://commits.webkit.org/250871@main> Reviewed commits have been landed. Closing PR #884 and removing active labels.