The spec that VTTRegion::setScroll throws a SyntaxError in dead code has been removed at https://github.com/w3c/webvtt/pull/28, and now it should ignore assignment of an invalid string. WebKit is failing the corresponding WPT: https://wpt.fyi/results/webvtt/api/VTTRegion/scroll.html?label=experimental&label=master&aligned&q=webvtt
Pull request: https://github.com/WebKit/WebKit/pull/17567
Committed 267828@main (8de0edf59961): <https://commits.webkit.org/267828@main> Reviewed commits have been landed. Closing PR #17567 and removing active labels.
<rdar://problem/115215509>