I guess we have to confront whether we think that HTML fundamentally is a string that happens to contain tokens that could be interpreted as a data structure, or whether we see it as a data structure that has a string representation.

I think it just happens to be that the string representation is one of the most familiar and accessible formats to many people.

I don't subscribe to the idea that HTML somehow fundamentally is a string. However, even if we see it as a data structure, to many languages, data structures are not at hand, while objects with parochial APIs are. So you end up with a flurry of different libraries with their own way of doing things, instead of "this is just data, I'll use my language's generic data manipulation tools to deal with this".

I'd say it's definitely a data structure, but one with a canonical string representation. It may not be fundamentally a string, but that representation is critical to how we convey it's meaning to such extent that it's used across languages and browsers as the method of transport.

JSON is conceptually simpler and there's still a lot of quirks between libraries for that already (e.g. null/undefined, integer/decimal representation, and large numbers). XML has more going on to start, and then you get all the different libraries inventing their own abstractions as you said and it picks up a lot of pitfalls. FWIW; I've messed around a lot with configuration languages and it's definitely hard to get right so I understand how this differences accumulate.

I think this is actually a strong argument against HTML templating, if you saw someone templating JSON files with Jinja you would think them mad but we're somehow okay with it when it's HTML.