Quoting Victor's description from Bug #24208:

The DOM Core API setAttributeNS allows the creation of attributes that have a non-null namespace URI, but no prefix.

A minimal example follows below.
var doc = (new DOMParser()).parseFromString("<test/>", "text/xml", null);
doc.documentElement.setAttributeNS("http://www.example.com/", "attr", "value");

The XML serialization algorithm specified below does not seem to handle this case.
https://dvcs.w3.org/hg/innerhtml/raw-file/tip/index.html#dfn-concept-serialize-xml

(searching for "XML serialization of the attributes" shows the relevant paragraph)

Firefox and Internet Explorer handle this case by generating prefixes for the attributes.

Blink and WebKit do not yet handle this case, but I'm working on it. http://crbug.com/248044