You can use /sandbox/ for writing and testing your own code. Nothing in /sandbox/ will get checked into the repo, except files that end in .example (so don't edit or add those files). To get started run `npm start` and open the combined-tracks.html

npm start
open http://localhost:9999/sandbox/combined-tracks.html

This player has the captions-only captionsButton, the subtiles-only subtitlesButton and the subsCapsButton which shows both kinds. Typically you'll use either just the subsCapsButton alone, or one or both of the captionsButton and subtitlesButton.