In my humble opinion, responsive design is a future. One layout, carefully designed, will look beautiful and by user-friendly, no matter, which device user will use to view it. Of course, you can craft responsive designed website by hand, but the easiest way is to use some framework. But what about non-framework elements, like for example images? How to style them, what size should they have, to be responsive and look exactly like other, framework elements, on any device?
There are at least few approaches to solve this problem, among which one or two are not that famous.