Before this commit, we tweaked the subtitle position by modifying the
VTT file, line by line with a regex because I did not know it was
possible to use CSS for it.
But apparently there are Shadow DOM elements that we can use instead.
This new approach improves:
- Wrapping long lines. Before, the text would go off the edge of the
screen. Now it wraps intelligently.
- The subtitles move up to get out of the way of the controls when
those are visible.
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