OverPicture is a macOS Safari 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.