chromium/third_party/blink/web_tests/css3/selectors3/xml/css3-modsel-171.xml

<?xml-stylesheet href="css3-modsel-171.css" type="text/css"?>
<test>
   <p xmlns="http://www.w3.org/1999/xhtml" xmlns:xhtml="http://www.w3.org/1999/xhtml">
    <span xhtml:class="fail">This should be green.</span>
   </p>
</test>