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 make a copy of index.html.example and rename it to index.html.
cp sandbox/index.html.example sandbox/index.html
npm run start
open http://localhost:9999/sandbox/index.html
This player has the captions-only CpationsButton, 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.