The namespace-uri() returns the namespace URI of a node.
string namespace-uri(node-set[optional])
Optionally, a node-set. If no argument is specified, the function uses the context node.
A string with the namespace URI of the expanded name of the context node, if no argument is specified, the namespace URI of the expanded name of the first node if a node-set is specified, or an empty string if an empty node-set is given. Only element and attribute nodes are not empty.
Defined: XPath, section 4.1.
NS support: Supported as of 7.0.
