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".