> You can make webapps without WebAssembly, without ECMAScript, or even without CSS. But you absolutely need HTML
You don't actually need HTML to make a webapp, even an interactive one. HTTP, CSS, and Firefox is enough.
Try this on Firefox: https://lyra.horse/fun/tic-tac-nohtml/
View-source for that page should give you an empty 0b file.
(and you can't argue about the browser creating a DOM anyways, because in that case you cannot have a webapp without CSS since the browser always applies its own agent stylesheets)