If you're on a Mac check out https://github.com/liquidx/webviewscreensaver - Works well but you might not always have an internet connection so I just saved the HTML/CSS/JS into a folder and linked directly to that (and it works!)

On windows, I imagine you could set screensaver to call firefox directly to load the web page - and set firefox to default to full screen mode.

http://kb.mozillazine.org/Command_line_arguments