Hm, alrighty then. Seems like an unnecessary distinction to me in that it deeply constrains what SPAs can be, but I'm not a webdev.

The rule is either "uses html to render" or an even looser "renders in the browser". That doesn't seem like a deep constraint to me. You exclude a couple very popular but historical plugins where the browser set up a rectangle and handed it off to an external piece of code, and pretty much nothing else.