Implement the use of <track>

[lepe]

Example:

<video-nd class="youtube" src="/video/demo-HD.ndjson" controls loop autoplay fps="30" autosize>
       <source src="/video/demo-QVGA.ndjson" width="320">
       <source src="/video/demo-4K.ndjson" width="3840" height="2160">
       <track src="/subtitles/en.txt" kind="subtitles" srclang="en" label="English">
       <track src="/info/data.txt" kind="descriptions" label="Data">
</video-nd>

It should place captions according to the documentation.