In a wiki environment allowing HTML is a very dangerous option. You should convert all the HTML code to wiki syntax or write some wiki-plugins to generate your HTML forms.
Why is this dangerous?
Well I could post a img src based JS attack on your site. to give me the contents of the tiki login cookie, and then use that data to gain control of the TikiWiki site.
You should always disable HTML on the wiki, and convert anything HTML to use wiki-syntax and wiki-plugins.