There's already "DF (Distraction Free for) YouTube", which does (almost?) the same and a bit more. Not sure if it's open source and it looks like it's not being maintained but it still works.

https://chrome.google.com/webstore/detail/df-tube-distractio...

https://addons.mozilla.org/en-US/firefox/addon/df-youtube/

Personally, I'd benefit more from a "Less Addictive HN".