A line: 0% positioning should set the box on top of the video viewport 1 (when alignment is top-left which is the default).
Fixes #29230
The webvtt sample in that issue also has "position: 63%" which is not respected but that's not the subject of that issue.
https://www.w3.org/TR/webvtt1/#webvtt-line-cue-setting ↩