Found a way to fully support XML capable browsers and still support Microsoft's.

RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} .*MSIE.*
RewriteCond %{REQUEST_URI} \.xhtml$
RewriteRule .* - [T=text/html]

- IBM

Browser usage statistics from chaosreigns.com

By browser.
By browser and version.

The numbers are week ending dates.

This was some very thorough work. 82 regexes, including the one for the wonderful standard for Mozilla / Gecko / Firefox based browsers.