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<?xml-stylesheet href="styles.css" … ?>

If the rule is applied, it's corresponding CSS above this paragraph should have a green background. Everything else is negotiable.

Notes: This prolog must be above the doctype, which means the stylesheet will have to contain rules with higher specificity than other stylesheets being used. Also, the markup must be sent by the server with the MIME type "application/xml". You could for example use the following PHP:

<?php
    header("Content-Type: application/xml; charset=charset");
    echo "<?xml-stylesheet href=\"styles.css\" type=\"text/css\" ?" . ">\n";
?>

Many browsers that can parse and display these filters properly require that you specify your character entities in numerical format; for instance, &#160; instead of &nbsp;