Youtube, and their removal of the 'auto play' button for (music) videos. - there's a fix, but it's a pain to do for every damn video.
Add to that, they create a 'playlist' that shoves stuff you probably don't want to watch/listen to into the mix - which also auto plays.
30 minutes of faffing around to find 2 extensions for Vivaldi that will hopefully give me back some control.