You can even get subtitles on top of HTML5 videos with simple positioned elements with text shadows for borders. I can't say I understand the motive of the project either, other than "see, everything can be done in JavaScript!"
Also, here's a fun quirk of the subtitles track element: you cannot remove a track once added.
Another obstacle for full FCC compliance: styling. You must allow users to style your captions. Captions are put in the shadow DOM - Firefox does not allow you to style the shadow dom.
Don't forget to add IE into the mix!
Doing this natively in JS via canvas allows you exactly that - rendering these things natively, in real time, with all the support for rare features you could ever need.