UA styles will also be handled through this `CustomCounterStyle` type, so the `Custom` part isn't relevant.
<rdar://problem/106736501>
Pull request: https://github.com/WebKit/WebKit/pull/11538
Committed 261676@main (2c2726ff90d9): <https://commits.webkit.org/261676@main> Reviewed commits have been landed. Closing PR #11538 and removing active labels.