I don't interpret "html and js had syntactically shared data structures" to mean what you wrote, unless you also replace all other JS data structures (arrays an

I don't interpret "html and js had syntactically shared data structures" to mean what you wrote, unless you also replace all other JS data structures (arrays and dicts and ...) with <> syntax. You've made it more concise but it's still distinct. For an example of a system where the programming language and document generator share data structures, look at Hiccup (and Reagent). One of their very first examples is putting a loop inside a list. There's no special API for the attributes -- it's just an ordinary map. There's no special API for the children -- it's just an ordinary vector.