I'd phrase it more that there is a document with mixed use items marked up throughout it. Some items in the document are code, in which case you probably want to fence the code with a marker on what language is used. Other items are just prose, in which case you'd like to just write the prose as much as you can.

Some items can even be other forms of xml that have their own schemas dictating what is valid. (Thinking SVG here.)

I'll also note that even there, I can see why HTML went with the odd parsing they do. XHMTL tried going with "well formed" documents, but that falls flat for the authors. Is why "sections" of a document are essentially just collecting all of the "h" tags and making an implied tree out of that. As opposed to making the tree directly. To that end, my markup language of choice for Jupyter style things is org-mode in emacs. Yes, it has some warts; but again, all formats that I have ever seen have warts.

Edit: I want to add that I don't intend this as a "correction." I should say that I agree with your post. Complicated field where I doubt I'd have done better than most others. :)