As an additional tip, if you want to get rid of the "You are now fullscreen" (or whatever it says) that comes across the top... you can set the following to 0

  full-screen-api.transition-duration.enter
  full-screen-api.transition-duration.leave
  full-screen-api.transition-duration.timeout
  full-screen-api.warning.delay
  full-screen-api.warning.timeout

these are all involved in the popup when you fullscreen a video