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.

Matt Martz

achmafooma

Moves the gallery shortcode styles to the head so it doesn’t break XHTML 
validation; allows disabling or modifying the default gallery styles.
By default when using the WordPress gallery, the styles are placed into the 
post content which breaks XHTML validation. This plugin moves the style into 
the head of the page using a look-ahead to determine if the [gallery] 
shortcode is used in any posts.
This plugin also gives you the option to modify the default gallery style 
CSS or disable the gallery styles entirely (so you can control it from your 
template CSS files).
This plugin uses ideas recommended in a patch located at 
https://trac.wordpress.org/attachment/ticket/6380/6380-style.diff
Special thanks to 
* The original author: Matt Martz, http://sivel.net 
* Serbo-Croatian (sr_RS) translation: Borisa Djuraskovic, http://www.webhostinghub.com 
* Spanish (es_ES) translation: Ogi Djuraskovic, http://www.webhostinghub.com/

Gallery Shortcode Style to Head

Moves the gallery shortcode styles to the head so it doesn't break XHTML validation; allows disabling or modifying the default gallery styles.

FacadeX vs Gallery Shortcode Style to Head