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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.

Weston Ruter

This plugin is developed at <a href="http://www.shepherd-interactive.com/" title="Shepherd Interactive specializes in web design and development in Portland, Oregon" rel="nofollow ugc">Shepherd Interactive</a> 
for the benefit of the community. No support is available. Please post any questions to the <a href="https://wordpress.org/tags/xhtml5-support?forum_id=10" rel="ugc">support forum</a>.
Allows templates to serve XHTML5 markup by adding the necessary JavaScript shim for IE7+, and for IE6 by converting all new elements to <code>div</code>s and <code>span</code>s with class names that contain the original HTML5 element name prefixed by “<code>html5-</code>“, for example: <code><article></code> becomes <code><div class="html5-article"></code> 
and <code><time></code> becomes <code></code>
If the browser supports XHTML, the content is served with the <code>application/xhtml+xml</code> 
MIME type. This can be disabled by placing <code>remove_filter('option_html_type', 'xhtml5support_filter_html_type')</code> 
in <code>functions.php</code>.
Note that print stylesheets do not get applied to IE7+ even with the shim, so if you are relying on the ability for IE users to print properly-styled pages, you must force the IE6 renaming behavior for all versions of IE by doing <code>update_option('xhtml_support_add_msie_shims', false);</code>

XHTML5 Support

Allows templates to serve XHTML5 markup by adding the necessary JavaScript shim for IE7+ and by converting all new elements to divs and spans with app …

FacadeX vs XHTML5 Support