The goal of this technique is to deliver optimized, contextual image sizes in responsive layouts that utilize dramatically different image sizes at different resolutions. Ideally, this could enable developers to start with mobile-optimized images in their HTML and specify a larger size to be used for users with larger screen resolutions -- without requesting both image sizes, and without UA sniffing. As soon as rwd-images.js loads, it tests the screen width, and if it's large, it inserts a BASE element into the head of your page, directing all subsequent image, script, and stylesheet requests through a fictitious directory called "/rwd-router/". As these requests reach the server, the .htaccess file determines whether the request is a responsive image or not (does it have a ?full query parameter?). It redirects responsive image requests immediately to their full size, while all non-responsive-image requests go to their proper destination through a URL rewrite that ignores the "/rwd-router/" segment.