I've been playing around with how different Web 2.0 sites handle HTML. What I've found so far is:
* Facebook doesn't take much, if any. I can't get forms to work.
* MySpace takes forms, but I can't copy in a favicon link
* Ning doesn't take the form tag. I got it to take the link tag and I got my favicon to work for about two seconds, but as soon as I reloaded the page Ning sripped it out.