Prefix Declared |
Text from specification
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.
Explanation
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.