I understand the issue of mixing state models.. angular SPA style vs HATEOAS.
But that still doesn't answer what I was asking about mixing presentation concerns into business logic.
Returning HTML means the presentation layer has to know about HTML. Why would that require the business logic to know about it?
So instead if returning data (JSON, XML, whatever), every part of your API now needs to return parts of client-side UI, and be aware of changes in it.