Fun, but entirely missing the point. You can't force a window to cover a window from an independent program, since you have no authority over that other program

Fun, but entirely missing the point. You can't force a window to cover a window from an independent program, since you have no authority over that other program. But obviously you can design a popup to cover other windows in the same program. The scope of this "top layer" is the web page - it does not enable you to force-close other tabs containing your competitors web site. Popups have existed on the web for ages. This just proves a more convenient syntax without having to inject dom elements at the end of the body element (which is how popups are traditionally done).