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.

tompahoward

The Tiny XHTML plugin re-adds the xhtmlxtras plugin support to the TinyMCE Rich Visual Editor. 
The TinyMCE table plugin is normally bundled with TinyMCE, but it has be removed from the 
version within WordPress. This plugin puts it back in.
The xhtmlxtras TinyMCE plugin allows you to control common attributes, such as class, id, lang, onclick, etc. 
It also provides buttons for creating cite, ins, del, abbr, and acronym elements.
Most of the credit for this plugin belogns to [Moxiecode Systems] (http://tinymce.moxiecode.com/ ), 
who wrote the Table plugin for TinyMCE, which this plugin uses.
<h3>Release Notes</h3>
<ul>
<li>0.0.2
<ul>
<li>Added workaround for TinyMCE compressor issues.</li>
</ul>
</li>
<li>0.0.1
<ul>
<li>Fixed defect in link to xhtmlxtras stylesheet</li>
</ul>
</li>
<li>0.0.0
<ul>
<li>Initial Release</li>
</ul>
</li>
</ul>

Tiny XHTML

The Tiny XHTML plugin re-adds the xhtmlxtras plugin to the TinyMCE Rich Visual Editor

FacadeX vs Tiny XHTML