Standard: http://www.w3.org/TR/xpath#function-namespace-uri

string namespace-uri (node-set?)

Standard excerpt:

The namespace-uri function returns the namespace URI of the expanded-name of the node in the argument node-set that is first in document order. If the argument node-set is empty, the first node has no expanded-name, or the namespace URI of the expanded-name is null, an empty string is returned. If the argument is omitted, it defaults to a node-set with the context node as its only member.