:) thanks for the review.
it doesn't work on autoplay videos because it's just a script tag injected into the page. an autoplaying video doesn't load a new page, youtube's javascript does a request and replaces the video element in the dom (and a lot of other elements ^_^).

i don't know how much JS you know, but you might be able to implement something as a callback to a mutation observer: https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver

:)