In addition to declaring an appropriate DTD in a document type declaration, a valid XHTML document must also declare an XHTML namespace in the html element. Following is the XHTML 1.1 namespace, which is associated with the DTD you just learned about:
http://www.w3.org/1999/xhtml
<html xmlns="http://www.w3.org/1999/xhtml">
...
</html>