If given argument is a node, the function returns the URI string of the namespace specified in the name of node. If the argument is node-set, first node of the node-set will be considered. Empty string is returned if the argument is an empty node-set, a node without name or if the argument is not a node.
If the argument is omitted, context node is used instead as if it is a node-set of one element.
String
Find all elements whose namespace URIs contain "html" substring
//*[contains(namespace-uri(),'html')]