Creating a default namespace

A useful base Default namespaces let you specify a URI for all of the elements in your document which have no prefix. If all or most of your elements are in the same namespace, this can make your documents less verbose and easy to work with.

An example from XHTML

<html xmlns="http://www.w3.org/1999/xhtml">
...
</html>

XHTML 1.0 uses only the default namespace The default namespace defined in the html element will be applied to all un-prefixed elements in the entire document. The specification includes an example that mixes another vocabulary with XHTML using prefixes, but XHTML 1.0 elements may only use the default namespace.