Mike Walsh

phpHtmllib is a set of PHP classes and library functions to help facilitate building, debugging, 
and rendering of XML, HTML, XHTML, WAP/WML Documents, and SVG (Scalable Vector Graphics) images 
as well as complex html ‘widgets’. It provides a mechanism to output perfectly indented/readable 
tags, and a programmatic API to generating tags on the fly.
More information can be found on the <a href="http://michaelwalsh.org/wordpress/wordpress-plugins/phphtmllib/" rel="nofollow ugc">phpHtmlLib</a> 
plugin page.

phpHtmlLib

The phpHtmlLib library contains a set of PHP classes and library functions to help facilitate…

edik

If FacadeX is active, the public WordPress sites are sent with the xhtml mime type. The internal sites like the administration panel and the login screen, which have many xhtml syntax errors, are still sent with the standard mime type.
You must ensure that your public sites have valid XHTML, otherwise you will get parsing errors in your browser.

FacadeX

Sends public sites with the XHTML mime type. The internal sites, which have many XHTML errors, are still sent with the HTML mime type.

phpHtmlLib vs FacadeX