> At the time this “markup” concept got everyone all excited. But it ended up not being the way to solve problems like what you seem to describe, which don’t have the arbitrary document aspect at all.

But it does have the "arbitrary document aspect" when you add the desire for site-local claims.

> JSON solves it by being a data structure first and last.

I'd say that JSON solves it by being remarkably simpler than XML and by having become ubiquitous.

Your points about HTML... keep in mind that HTML is for humans, but what we're talking about here is for programs.