According to <http://dom.spec.whatwg.org/#dom-document-createelement> (20 March 2014), document.createElement() should always return an element in the HTML namespace: [[ The createElement(localName) method must run the these steps: 1. If localName does not match the Name production, throw an "InvalidCharacterError" exception. 2. If the context object is an HTML document, let localName be converted to ASCII lowercase. 3. Let interface be the element interface for localName and the HTML namespace. 4. Return a new element that implements interface, with no attributes, namespace set to the HTML namespace, local name set to localName, and node document set to the context object. ]] At the time of writing, document.createElement() [1] returns an element in the HTML namespace for an XHTML document. Otherwise, it returns an element in the "null namespace". [1] <http://trac.webkit.org/browser/trunk/Source/WebCore/dom/Document.cpp?rev=166600#L854>
For completeness, I ran into this issue when merging a Blink patch in bug #131079.