Windows used to secretly use green screens to render videos, which is how you could trick MS Paint into becoming a video player
via tenor.com
Short excerpt below. Read at the original source.
If you opened up the ol’ Windows Media player back in the 95, 98 or XP days, brace yourself for a mild shock: it was lying to you. And by lying, well, what I really mean is rendering video somewhere other than inside the actual window that was open on your desktop—sort of a parallel […]