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 focus-visible.html

npm start
open http://localhost:9999/sandbox/focus-visible.html

This demo shows how to implement polyfill for the :focus-visible selector. You can read more about the polyfill here.