Why can't this adaptation happen on the server? Client tells server what the resolution possibilities are ("the current browser size is X by Y, and my screen size (and thus maximum browser size) is W by Z), and server gives back only styles relevant to that size. On mobile devices where the browser is always fullscreen, that means a maximum of two layouts (horizontal, vertical).
Because clients don't send this information to the server (and if they did, its validity would be suspect), and because browser layout engines and CSS are already designed to be independent of specific device metrics. It's a solution to a problem that doesn't need solving.
It sends the User-Agent, which can be used to determine whether it's the desktop or not. It doesn't solve the portrait and landscape changing problem. So some client-side responsive design would still need to be used.
I think a hybrid approach would be too tedious for someone developing a website to do, in order to get the payoff of sending less to mobile browsers, but it may be worth implementing in a framework.
2) A user agent doesn't encode any information about the actual metrics of the device.
Doing layout by user agent is basically a worst-case hellhole of browser-specific codebase fragmentation. There's a very good reason why best practices have been trending away from "test for vendor class" and towards "test for agent capabilities".
Well a sufficiently complex framework could deal with it, by making another trip to the server if it discovers the code being sent down the first time isn't good enough.
The fact that I'm on a desktop does not take into account that my browser window may be smaller than my 'desktop screen'. Responsive is the way to go because it works.
I didn't mean to imply that this is possible with technology we have at the moment. A piece of JavaScript code (framework, perhaps) could surely send the needed data, and for those with JS disabled, the page could fail over to using media queries (to be exact, it would need to load the media query version the first time your site is loaded, but could immediately redirect to the lighter JS version if you have JS enabled and the light/JS version would be loaded for future requests).
