[Catalyst] Validation of entered content

[John M. Dlugosz]

I have a form where an admin can change a part of the content on a page.  The form lets 
someone do it without having to update the files on the server in a more traditional 
manner; BUT, it must be correct XHTML.  If the admin types a <p> or a stray '<' for that 
matter, it will render the resulting page not well-formed.

Combine that with the feature of serving XHTML to browsers that accept it, and the site 
breaks on browsers other than IE.

What's a good way to validate something programmatically before committing it?  Better 
yet, is there a rich editor or wiki- or phpbbs-like translator that I might use for 
soliciting the input that I might use instead?

The current content has a couple paragraphs and a bulleted list.  So it's not just a 
simple blank, but must allow an amount of rich content.