Heh, sorry for making you jump the hoops @jan
The weird formatting is mostly because this is how Gutenberg wants to format videos. Hacking various sources into this is how I did it before Gutenberg, that would still not allow me to offer different formats (or subtitles). So this is basically a combination of both snippets with the hope that Gutenberg will eventually understand this.
I was delighted to see that Gutenberg would offer an additional CSS for videos now alas it’s added on the wrong tag and you picked exactly that test. Usually I put this manually on all sources. See https://beko.famkos.net/2021/05/15/playing-an-old-sound-box/ for example.