Interesting. But it has one flaw; how exactly would I do this curl https://api.twilio.com/2010-04-01/Accounts -H "Accept: application/json inside a browser on a normal GET request?
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.