error on W3C specification page / question

Dear W3C site editors,
 
I am preparing an XHTML 1.1 document, and fount that the example given on a
page cannot be validated (either by the old or the beta validator):
http://www.w3.org/TR/xhtml11/conformance.html#strict
 
I suggest you correct this error on the page.
 
Because of this, I do not know how to do what this example shows.
How can I add other namespaces to an XHTML 1.1 (or 1.0) document?
 
Thank you in advance,
 
Géza Kiss
Budapest University of Technology and Economics
Department of Telecommunications and Media Informatics