OverPicture for Safari is a App Extension that allows you to play any HTML5 video with the new picture-in-picture mode.

Press the play/pause button in any web video player in Safari and then in the toolbar PIP button and the video will start playing in picture-in-picture over your windows. And it will even embed custom buttons in some video players like YouTube.

Note: You have to open the app and then enable the Safari app extension in Safari preferences.