The short answer is that browser vendors need to get their act together. They've shirked this because no-one's demanded it. Fortunately until they do, broadly,

The short answer is that browser vendors need to get their act together. They've shirked this because no-one's demanded it. Fortunately until they do, broadly, a browser user will always want the newest resource, and the type can usually be defaulted (you're using a browser, so you want HTML by default where available). Additionally, XHR and related technologies usually allow adding custom headers.