@Celaira: For security and performance reasons, I'd advise against it...
Could someone break something fundemental with what I was thinking, Shien?
Yes. Allowing HTML/CSS also allows for Javascript, Flash and Java to be embedded. These three are common attack vectors for malware/spyware/etc. That's not to say that all Javascript, Flash or Java is evil, but you're basically allowing anyone who can make an account (and if you recall, we got a fair few number of spammers last year on the Guild 2.0, so the machines can easily adapt) to inject these technologies into a post. That's a huge security hole, and as much as I'm sure Mahz would like to trust the Guild not to do that, all it takes is one person doing so intentionally or by accident to do a lot of damage.