Some notes, while I was working on bug 24208:

Firefox-as-reference:
"<xml:div/>" == new XMLSerializer().serializeToString(document.createElementNS("http://www.w3.org/XML/1998/namespace", "foot:div"));

* "xml" prefix trumps actual prefix for nodes in the XML namespace in the XML Serializer.
(Same goes for nested elements with XML namespace:
...appendChild(document.createElementNS("http://www.w3.org/XML/1998/namespace", "toe:span"));
"<xml:div><xml:span/></xml:div>" -- XMLSerializer
"<foot:div><toe:span></toe:span></foot:div>" -- HTML Serializer uses QName

Per DOM4:
* XMLNS is tightly controlled (reserved) via createElementNS -- prefix/QName <-must match-> namespace
* XML prefix -> XML namespace is controlled (reserved), but not vice-versa. 

Possible cases to handle:
"<xmlns:div/>" == new XMLSerializer().serializeToString(document.createElementNS("http://www.w3.org/2000/xmlns/", "xmlns:div"));
"<xmlns xmlns="http://www.w3.org/2000/xmlns/"/>" == new XMLSerializer().serializeToString(document.createElementNS("http://www.w3.org/2000/xmlns/", "xmlns"));
* ASSERT: Follows generic serializer rules with regard to expansion/prefix use.