The only html5 think here is border-radius. Now this just shows that we can encode weird shapes using borders which is a well known fairly old technique.

Perhaps we can have a small image maker took which generates html5 vs an image.

Now that is great and all and is probably a really nice experiment into doing stuff with CSS, but this is a tiny fraction of what CSS can do being used, but practically, an image takes less memory/cpu to render.