See:

• Namespaces in XML, 14 Jan 1999.

The prefix xmlns: was specified as a syntactic device for declaring namespaces, but was not itself associated with any namespace name by the Jan 1999 namespaces specification. But in some processing contexts, e.g. DOM, it is useful to represent all XML attributes as (namespace name, local name) pairs. For this purpose, the namespace name http://www.w3.org/2000/xmlns/ is assigned.

Note that you must use the reserved prefix xmlns: when declaring namespaces. It does not work to declare another prefix with this namespace name and then try to use it for namespace declarations.

The attribute name xmlns, which is used to declare the default namespace, is also hereby associated with this namespace name.