`appearance: slider-horizontal` behaves like `appearance: auto`, and only applies to <input type=range>. `appearance: slider-vertical` should too.
<rdar://problem/104147398>
Pull request: https://github.com/WebKit/WebKit/pull/8555
Committed 258924@main (b03a814cfb39): <https://commits.webkit.org/258924@main> Reviewed commits have been landed. Closing PR #8555 and removing active labels.