Responsive Web Design is an extremely important approach/technique/movement for making web mobile-friendly. Given that web mobile traffic is exploding and there were more smart-phones sold in 2011 than PCs the importance of the movement can not be emphasized enough.

We have some significant obstacles on the path of fully optimized Responsive Web Design, however. Responsive Images (smaller images for smaller screens to optimize download times) and optimized CSS/JS (mobile devices don't need the same JS/CSS as desktop browsers do) are the obvious ones.

The most optimal way to handle responsive images and optimize CSS/JS would be on the server-side. However, server-side does not have enough information about device capabilities, resulting in emergence of all kinds of cruft-y solutions (e.g. using div's for img tags etc.) that should not exist.

However, browsers do know a lot about devices they are running on and could pass that information to servers as part of the initial request, making it possible for servers to get way smarter than what we have now.

Something as simple as if browsers passed along device's width/height information as part of the initial request headers would go a very very long way, making it possible to make a lot of intelligent decisions on the server-side (eventually allowing "media-queries-like" systems on the server-side).

Since some proxy servers cut-off or disable custom headers, probably the safest way to add this is to add width/height information in the user-agent header.

The text of this plea was submitted to all major browser vendors and WhatWG group. Many thanks to @paul_irish for suggesting the latter.