All of that said, there is a lonely specification at IETF about HTTP transparent negotiation. Basically it is a kind of mediaqueries for the server. It gives information on what type of resources are available on the server, so the client could select the one which is appropriate for its context. I wrote about that. http://my.opera.com/karlcow/blog/2011/12/08/responsive-images-and-transparent-content-negotiation-in-http
It is not necessary an immediate solution because it has the drawback that most front-end Web developers have no control or sometimes knowledge about the back-end, and content negotiation had a bad publicity in the past among browser implementers.