data:text/html,<input type="range" style="writing-mode: vertical-lr;"> should work as expected. Code from `appearance: slider-vertical` is probably re-usable here, although it's worth noting that this renders a slider advancing in the opposite direction from the inline direction.
<rdar://problem/102598641>
WPTs being added here: https://github.com/web-platform-tests/wpt/pull/37131
Pull request: https://github.com/WebKit/WebKit/pull/6778
Committed 256994@main (7abec143ddb9): <https://commits.webkit.org/256994@main> Reviewed commits have been landed. Closing PR #6778 and removing active labels.