Hmm, is opening a fullscreen video considered opening a new application, or is it considered a single application (the browser) displaying different content? I thought it was the second case, so multitasking I don't think applies. Same for desktop chrome, when it shows a fullscreen video, that's not a new application, just the browser displaying content in a new way.

For picture in picture mode, I don't think multiple fullscreen videos should be a valid configuration of picture in picture.