ToT r439712

Chrome fails the WPT custom-elements/parser/parser-uses-registry-of-owner-document.html "HTML parser must use the registry of window.document in a document created by document.implementation.createXHTMLDocument()" (the test has an odd name, but it is actually doing createDocument with an XHTML NSURI.)

The HTML spec has this nugget [1]:

"This Document must then be populated with DOM nodes that represent the tree structure of the input passed to the parser, as defined by the XML specification, the Namespaces in XML specification, and the WHATWG DOM standard. When creating DOM nodes representing elements, the create an element for a token algorithm or some equivalent that operates on appropriate XML datastructures must be used, to ensure the proper element interfaces are created and that custom elements are set up correctly."

Note the "custom elements are set up correctly" part.

[1] https://html.spec.whatwg.org/#xml-parser