There's the picture in picture stuff when you navigate away too. I recently did that with a MPA, and it was a not straightforward experience to get right.
It's kind of an awful experience though? Do people actually want their videos to follow them? If I'm navigating away it's because I'm done, it actually makes me kind of angry that the video chases me.
In my case it was a podcast player, so primarily audio, where playing in the background is a perfectly normal thing to do and you might want to browse other content while playing.
Maybe such functionality is best left to the browser itself. Firefox already has the functionality to "detach" a video. Then you can scroll wherever you want and still see that video.
I still think you are not too wrong though. I usually use invidious and there are no interactive widgets, except for the video player, which I think is default HTML5 player. I rarely need anything else. And PIP can be done in Firefox with ease, without the website needing to implement anything.