This plugin is based on the HTML Purifier library ( http://htmlpurifier.org/ ), and has been ported to joomla by http://www.joomlashow.it/ staff.
This plugin purifies html code of contents to gain valid XHTML.
It's just 0.2 version but we want to make a well-working extension, our goal is to make possible a total xhtml 1.0 strict compliance with joomla 1.5.
We need help testing and getting feedback.

There are 4 versions available:
Html purifier Mambot - Joomla 1.0 - PHP 4 based on 2.1.3 version of the HTML Purifier library;
Html purifier Mambot - Joomla 1.0 - PHP 5 based on 3.0 version of the HTML Purifier library;
Html purifier Plugin - Joomla 1.5 - PHP 4 based on 2.1.3 version of the HTML Purifier library;
Html purifier Plugin - Joomla 1.5 - PHP 5 based on 3.0 version of the HTML Purifier Library;

Plugin allows configuration of the following parameters:

Tidy Level: High, Medium, Heavy;
Doctype: Xhtml 1.0 Transitional, Xhtml 1.0 strict, Html 4.0 Transitional, Html 4.0 Strict;
Linkify: converts automatically every strings http: // in a link.
Trusted HTML: if setted "no" it doesn't permit to use script and style tags.
Proprietary CSS (PHP 5 only): if setted "on" permits to use of non standard css properties.
If you have some problems in a particular content item you can deactivate the plugin simply writing {disablepurifier}.

And don't forget to thank Edward Z. Yang for the library!

ADD:

Thank you for your appreciation.
I noticed a little problem on a site of mine, the plugin cutted the apostrophes, but really don't know why only on that site.
Anyway if you find problems please report!
This module is Joomla 1.5 also compatible.