I noticed the Picture-in-Picture button for video is not available on Facebook, either. (At least not on the new Beta interface on Facebook Live videos being watched after the fact, i.e. no longer live). Presumably it's the same problem, where Facebook is not actually using an HTML5 player.
@Gwen-Dragon nope, same thing. Which kinda makes sense, because except if not for some strange VSYNC/G-Sync double stacking bug, both of those technologies are supposed to fight against tearing.
The problem is that Vivaldi somehow manages to negotiate this custom 25fps mode, then while showing YT interface renegotiate it back to 60 (still probably in G-Sync, just maxed out) while drawing 25fps video. Actually i don't really know how all this stuff works, it's just my crazy theses. As i said, both Opera and Y.Browser managed to stay out of it and played back at 60 without problems.
But i've made some videos demonstrating the effects: without and with --disable-gpu-vsync.
Also Vivaldi using G-Sync is not a given thing. After exiting the game it didn't trigger it until PC restart, nor when i tried to record playback with Nvidia's Shadowplay.