Inserts a single Attr object into the current collection of attributes. If the destination of the attribute is an existing element, you may also use the setAttributeNodeNS( ) method on the element to insert the Attr object. When the setNamedItemNS( ) method is invoked, the browser first looks for a match between the new attribute's pairing of local name and namespace URI and existing attribute local names and namespace URIs within the collection. If there is a match, the new attribute replaces the original one; otherwise, the new attribute is added to the collection.