There is an app or two floating around that allows this. They also allow using screen savers, videos, etc. as backgrounds also. The downside is they'll eat up resources fast.
Backlight doesn't do HTML, but will do screen savers nicely (not a resource hog, but still eats some up).
The one that probably fits your need the best is called DesktopMagic. It allows HTML/web pages, videos, screen savers, etc. to be used.
Wow, sorry for the misinformation! I had no clue those applications existed. I guess I should take my own advice of "simple searches on versiontracker.com" before I open my mouth...