Prefix Declared
The namespace prefix, unless it is xml or xmlns, must have been declared in a
namespace declaration attribute in either the start-tag of the
element where the prefix is used or in an ancestor element
(i.e., an element in whose content the prefixed markup occurs).
The prefix xml is by
definition bound to the namespace name http://www.w3.org/XML/1998/namespace.
The prefix xmlns is used only
for namespace bindings and is not itself bound to any namespace
name.
You have to declare all namespaces before you can use
them. The prefixes have no meaning without the declarations, so
using a prefix without a declaration context is an error. The
namespace with the prefix xml
is permanently defined, so there is no need to redeclare it. The
xmlns prefix used by
namespace declarations is not considered a namespace prefix
itself, and no declaration is needed for it.