They were implemented in bug 142154, to be used in bug 146250, but that never got fixed, and now modern-media-controls have taken over.
<rdar://problem/93703906>
Pull request: https://github.com/WebKit/WebKit/pull/881
Committed r294641 (250865@main): <https://commits.webkit.org/250865@main> Reviewed commits have been landed. Closing PR #881 and removing active labels.