It's actually possible to do away with the HTML altogether (or rather have a 0-byte HTML file) by using the Link HTTP header to specify the stylesheet - http://www.w3.org/TR/html401/present/styles.html#h-14.6

There was a fun demo that showed exactly this, but I can't seem to find it now. This page - http://annevankesteren.nl/test/html-element/style-header.php - uses Link header but also uses some HTML.

[edit - fixed second link]