Use Sierra Picture-in-Picture for Unsupported Videos by Double-Clicking the Right Mouse Button
Picture-in-picture video is a great feature of macOS Sierra , but only a handful of videos on the Internet seem to support it. One big thing is missing – YouTube. However, iDownloadBlog indicates that you can force the video to picture-in-picture conversion by double-clicking the right mouse button.
Just right-click (or Ctrl-click twice) on the video and you will see the Enter picture-in-picture option. As soon as you click on it, the video will go into PiP mode just like anywhere else. This option will be grayed out during ads, but otherwise it works as it should.
How to Use Picture-in-Picture for YouTube Videos in macOS Sierra | iDownloadBlog