I think you've missed the point being made here. What's being said seems to be that regardless of how much scripting/API calls there are, if the base is HTML then it is HTML.

To paint this another way, let's say you have UI toolkit "X" which defines elements using YAML but also provides a programmatic API in your favorite programming language to add, remove, modify, or otherwise modify elements. Now let's say there is another UI toolkit "Y" which defines elements using XML but, like toolkit X, it also provides a similar API. Regardless of how many YAML elements you explicitly create you've used X and not Y, in fact you've used YAML and not XML. It doesn't really matter if >50% of your code is using the API or not, just that you've used some amount of YAML.

That's exactly the situation here: the existing debugger interface is using XUL and this new version is using HTML. Even if there wasn't a single line of HTML code, it would still be using the DOM to generate HTML and not XUL--and that's the distinction being made.

Web browsers are HTML engines, so almost everything that runs in [the document frame of] a web browser is indeed HTML. You cannot under any normal circumstances get a browser to execute JavaScript outside the context of a document – even in the Developer Console. You cannot create a Canvas (which is an element, as in HTML element) outside the context of a document.

That is why to create one you do:

    document.createElement('canvas')

and there is no other way. `document`, by the way, being none other than:

    > document.constructor
    function HTMLDocument() { ... }

HTML is more than just the syntax. :)