If the namespace attribute is present, then it also is interpreted as an attribute value template. The string that results
from instantiating it should be a URI reference. It is not an error if the string is not a syntactically legal URI reference.
If the string is empty, then the expanded-name of the attribute has a null namespace URI. Otherwise, the string is used as
the namespace URI of the expanded-name of the attribute to be created. The local part of the QName specified by the name attribute
is used as the local part of the expanded-name of the attribute to be created.
Examples (xslt:xml): Overview 21:6