> Those exist?

For sure. Most of the backends we write only return JSON. If you request HTML or any other content-type, we return nothing. We just don't implement it.

If you write your backend from scratch and only intend for it to be used only with your web front-end, you can make it return bits of HTML (I have several applications in jQuery that work like this -- the value of the response just gets inserted via ("#id").html(val)).

Our current backends are written to support multiple generic consumers of which a web front-end is only one.