One word. Hackers. We want our systems to stay up so you can enjoy them, so don't currently allow use of html on the boards.
You can still post links and make text bold, italic or underlined using BB Code. If you've used message boards before, you're sure to have come across it. It's simple to use and less accident prone than html.
Just use the following tags:
Bold
[b]Add the words you want in bold here[/b]
Italics
[i]Add the words you want italicised here[/i]
Underlines
[u]These words will be underlined[/u]
Links
[link]Add a web address here[/link]
[link=web address]If you'd like others to click on a word like "clicky" use this format[/link]
[link=mailto:email address]Makes a clickable email address[link]
BB Code has a lot more tags than this, but the ones above are all that's available on our boards right now. If you'd like to use more then let us know.