For some additional insight on accept based conneg. The guys who created it, really wish they hadn't.

http://www.alvestrand.no/pipermail/ietf-types/2006-April/001...

"Regarding proactive negotiation in HTTP/2, I'll note that Waka strips all negotiation fields. I find the entire feature revolting, from every architectural perspective, and would take the opportunity of 2.x to remove it entirely." Roy Fielding http://lists.w3.org/Archives/Public/ietf-http-wg/2013JanMar/...