make youtube videos full-window in Safari

One of my favourite Chrome extensions is YouTube Windowed FullScreen that simply hides all the UI in Youtube and makes the video the size of the containing window. Given how weird MacOS full-screen is (fully modal! bizarrely slow and complicated transitions!), this works great for watching something without making your computer unusable for anything else.

Seemingly no one has re-implemented this for Safari, but Good Ol' CSS hacks save the day.