<person xmlns="http://www.java2s.com">
<name/>
<paragraph xmlns="http://www.w3.org/1999/xhtml">This is XHTML</paragraph>
</person>
http://www.java2s.com is the default namespace for the document as a whole,
http://www.w3.org/1999/xhtml is the default namespace for the <paragraph> element, and any of its descendants.
http://www.w3.org/1999/xhtml namespace overrides the http://www.java2s.com namespace.