Firefox Picture-in-Picture
PiP is such a great feature.  I have no idea how easy/hard it was to implement but it feels nice to use.  It does exactly what you'd expect it to do and nothing more.<p>I use it while watching overwatch league games in the browser because it allows me to resize the popped-out video arbitrarily, instead of the site's  embedded youtube player options of either tiny-window-in-browser or full-screen sizes (there is no theater mode in the embedded player for some reason).