Thanks for your replies both. It's not just using </div>, I can place <script> tags too, which work when loading the page. I know you can do this with templates too, but boards should have a check IMO.
1.Only admins can create/edit these things, so if somebody wants to "destroy/hack" something, he will be able to do this even the data are validated...
2. IMO it's really usable that we're able to use HTML there