See the TextTrackCue ctor & cue + cuebackground attributes spec text at the WHATWG: https://github.com/whatwg/html/pull/9771
<rdar://problem/116002871>
Pull request: https://github.com/WebKit/WebKit/pull/18176
Committed 268644@main (ed93c318ea69): <https://commits.webkit.org/268644@main> Reviewed commits have been landed. Closing PR #18176 and removing active labels.