Murray Altheim; Shane McCarron. <a href="http://www.w3.org/TR/2001/REC-xhtml11-20010531"><cite>XHTML&#8482; 1.1 - Module-based XHTML.</cite></a> 31 May 2001. W3C Recommendation. URL: <a href="http://www.w3.org/TR/2001/REC-xhtml11-20010531">http://www.w3.org/TR/2001/REC-xhtml11-20010531</a>