not sure what you're doing, but if you're in safari and trying to get web video to stay on top, once the video is going, in the location bar, right click the speaker icon, and enable PiP, Or I think new this year, safari has a video view mode, similar to reader view, same shortcut even (cmd-shift-R) which will get rid of all the UI, and just make it full window video, and if you cover the window, it automatically pops out to PiP