JSON APIs are typically more general. Here, you'll need different endpoints/handling for every flavor of HTML you may show that uses that same dataset. It's not so simple as Accept headers unless you always use exactly the same html, in every context on your site, to represent a "user with ID <foo>"
Not at all. Post a form submission, send back either the same form with errors displayed, or simply return out-of-band updates to error message divs. You can redirect from the response as well using htmx, so if the form submission is successful, you can redirect to a success page ... or simply update a div with a success message.
If you get a request asking for json, you just return a json with the same thing. Not much changes with API design if you were designing the API well in the first place.
