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

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).