Better than nothing
It seems safari forces fullscreen mode for vids to be portrait, there is no way around it with the built in fullscreen. This app will let you rotate sideways with the screen lock but even then if you manage to get the fullscreen button to show up in the sideways orientation, it will still auto switch to vertical. As far as I can see the best solution is to use this and hide the bar at the bottom by pressing Aa. Not ideal though.