The picture was one of the most exciting new features of Safari in macOS Sierra. It's quite convenient to be able to watch a video that is floating above everything else. It's compatible with a variety of major video streaming services.

The Enter Picture-in-Picture button can be found in the bottom toolbar on sites like Vimeo. To access the shortcut menu on YouTube, right-click and then right-click again. Select Enter Picture-in-Picture from the drop-down menu. The video will start playing right away. After that, you can move it to any corner of the screen. Click the little Quit button to the left of the play button to exit Picture-in-Picture.