Just a couple of silly experiments on a Sunday afternoon.
I think it's beautiful to overly animated GIFs on top of one another. If the topmost GIF has a transparent background it becomes hypnotic to see the synchronisity which appears to develop - akin to listening to Dark Side of the Moon while the Wizard of Oz is on the TV.
It's also possible to overlay GIFs to use as sprites. In this example, there's an animated background image (I got bored moving the clouds!), the ? blocks are separate GIFs positioned with CSS, as are the Goomba and Mario.
If you click on the frame, you can control Mario with your left and right cursor keys.