Youtube is the worst, because not only does it autoplay, but for the first 2s or so it's faffing around with JavaScript so no matter what you do in that time it will subsequently start playing. Like, it'll show up as "playing" and you pause it, then leave the tab, and like a second later the video will start despite your having paused it.

(for the record, at least half the time I open a youtube video I don't want it to play immediately)