Altering the presentation layer is possible precisely because HTML is a semantic API definition: one broad enough to enable self-description across a variety of domains, but specific enough that those applications can still be re-contextualized according to the user's needs and preferences.

Your point would be much stronger if all web forms were served in pure HTML and not 95% created by JS SPAs.

I think the web itself would be stronger if it was served in pure HTML and not 95% created by JS SPAs.