An example:
Flux CMS Wiki
"Flux CMS is a XML/XSLT CMS based on PHP 5 (
http://www.php.net/) and Popoon (
http://popoon.org/).
It's easy to use for the enduser and has WYSIWYG editing capabilites, but it's also very extensible and powerful for implementors to suit your needs.
And it's open source and free (GPL licensed)".
Related link:
Bitflux Editor - The Wysiwyg XML Editor