The code at http://hg.mozilla.org/mozilla-central/file/4d5760a4f96f/content/xml/document/src/nsXMLDocument.cpp#l123 is wrong per HTML5 http://www.whatwg.org/specs/web-apps/current-work/#html-documents HTML5 requires documents created with createDocument not to have the HTMLness bit set. Neither WebKit nor Opera sets the HTMLness bit from doctype. (Also: HTML5 requires all document objects to implement HTMLDocument and SVGDocument.)
Webkit creates XHTMLDocument only if namespace uri of the root element is XHTML. That is pretty wrong. Not sure what Opera does.
And Gecko uses doctype for document creation, because that is what should be used based on DOM 3 Core. I don't know why HTML5 doesn't allow creating HTML documents, only XHTML documents.
Per actual DOM this method should always create XMLDocument object, without additional treatment for the doctype. Other browsers works like that (I test Chrome, IE11, Opera-Presto), and only Firefox is still inconsistent here (and for example, can output HTMLDocument or SVGDocument). https://dom.spec.whatwg.org/#dom-domimplementation-createdocument I catch only one case, when Opera-Presto return HTMLness, but other browser still don't do this: <script> alert(document.implementation.createDocument("http://www.w3.org/1999/xhtml", "html", null)); </script>
I'm not sure if we want to do this without fixing bug 897815. Making the document not HTML doesn't change much in spec-land, but for us it means you lose all the HTMLDocument-specific properties like .body.
Or, alternatively, if DOM issue 217 <https://github.com/whatwg/dom/issues/217> is resolved and we implement it, it should also be okay to fix this, because passing an HTML DTD but not the HTML namespace is silly and it probably doesn't matter what we do in that case. An example of a wpt test that newly fails if we implement this as-is is dom/nodes/Node-isEqualNode.html, which tests that .createHTMLDocument() returns the same as a suitably adjusted .createDocument().
(Try run for reference: https://treeherder.mozilla.org/#/jobs?repo=try&revision=f8ba1feb3c25)