I use a DTD in my HTML to allow the special XML attributes Creative Commons includes in their anchor elements. Specifically, I use
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN"
"http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">
<!-- <!DOCTYPE html> -->
<html lang="en">
<head>
<title>Error :(</title>
</head>
<body>
<p>Hi! There's an error :(</p>
</body>
</html>
I'm pretty sure the above HTML is W3C-compliant, but the extension throws an illegal argument error, viewable in the "RUNTIME STATUS" tab of the extension.