Picture-in-Picture mode, originally included on the iPad Air 2 and iPad Pro, has now spread to macOS Sierra. This trick is not exclusive to YouTube and functions on any HMTL5 video source.
Right click once and allow the first menu to appear and then right click again. The macOS menu should appear and you will be able to choose Enter Picture-in-Picture and have a hovering video appear.
HTML5 video has this feature built in, and Vimeo is the first to support it on its proprietary player.
|You can click the shrinking window button in the bottom right (next to the fullscreen button) and access Picture-in-Picture|
Just like in Vimeo, you can click the shrinking window button in the bottom right and access Picture-in-Picture
|The right click to access Enter Picture-in-Picture works here.|
As little as I view Dailymotion, it is no exception. Right clicking the video and selecting Enter Picture-In-Picture works as well.
Hopefully these options work for you with whichever video source you choose. Keep in mind that Flash video is not compatible with Picture-in-Picture. Netflix and Hulu will most likely add support in the fall with the public release of macOS Sierra, but for now both have locked-down video sources.