I am taking comments on a web forum from arbitrary people. The interpretation of the HTML occurs at the user's browser. A lot of people will be using outdated browsers (IE 5.5 / 6), ergo security (at the source) becomes my problem. I cannot force them to upgrade their browsers.
I think this is very wrong for two reasons. First of all, the more web sites care of old browsers, the later people will upgrade them, therefore preventing the progress in Web (though IE 5.5 is not THAT old and bad, so this argument is not so strong). In Russia we some times say that a user with an outdated browser is an EPTH (Evil Pinocchio To Himself, don't ask me about source of this term). Secondly, I don't think that filtering HTML coming from an arbitrary user is a good idea. HTML is not very human-readable and too complex to achieve real safety without lots of work. My suggestion is to use some home-grown wiki-like syntax - it's easier to enter (*bold* instead of <b>bold</b>), easier to read (and your users would sometimes read their comments before posting - to check correctness), and easier to process, since it can't have security holes you're not aware of. But you're right, we are off topic.