Dysfunctional HTML in the sense that interactivity is disabled, but visually it is rendered and therefore you can see the proper webpage immediately.

Just compare the two cases, assuming 100ms for the initial HTML loading and 200ms for JS loading and processing.

With full JS, you don't see anything for 300ms, the form does not exists (300ms is a noticeable delay for the human eye).

With frameworks such as Astro, after 100ms you already see the form. By the time you move the mouse and/or start interacting with it, the JS will probably be ready (because 200ms is almost instant in the context of starting an interaction).

This is not new at all, old school server side processing always did this. The advantage is writing the component only once, in one framework (React/vue/whatev). The server-client passage is transparent for the developer, and that wasn't the case at all in old school frameworks.

Note that I'm not seeing this is good! but this is the value proposition of Astro and similar frameworks: transparent server-client context switching, with better performance perceived by the user.