the top bar and bottom bar do not disappear when in full screen mode 90% of the time in Safari 10.1.2 on MacOS 10.12.6. some times it disappears after a few minutes of playback. sometimes restarting safari helps, sometimes reloading the page helps.
This is a known issue and will be fixed next release.
In the mean time if you move the mouse over the controls then back off the controls they should hide as expected. If for some reason this does not work
alt-tab will fix it.
holy crap, thats a easy work around
thank you @chrishoage