Modern HTML, per the HTML spec, has implicit namespaces: if you look at the DOM on any site, you'll find the root element of any HTML page is an element in the "http://www.w3.org/1999/xhtml" XML namespace (c.f. `document.documentElement.namespaceURI`).

It was _absolutely deliberate_ that the DOM produced by parsing HTML and XHTML is now the same in overall structure, with the same XML namespaces, as it means browsers don't need to have things like `if localName == "html" and (namespaceURI == "http://www.w3.org/1999/xhtml" or isHTMLDocument)` all over the place.

Modern WHATWG specs put a lot of effort into reducing the number of places where the behaviour of a given DOM depends on whether the document is an HTML document—and most of those are places where non-namespace aware APIs default to the "http://www.w3.org/1999/xhtml" namespace in HTML documents.