If you use responsive CSS, what am I missing?
Responsive is about responding to container characteristics, instead of relying on the assumption that a certain known device has certain characteristics. Even with responsive, the rabbit hole goes waaay down, but at least you're avoiding the ghetto effect of "supporting" a set list of circumstances.
* Resizing your browser window.
* Changing font-size for accessibility (Ctrl+, Ctrl-)
* Moving your browser between multiple screens
If responsive was server-side, each of these scenarios would cause a page reload in order to accomodate the adjusted layout.
Responsive design doesn't just mean CSS media queries. It more commonly means using floating div/ul instead of tables for layout.
I love responsive design for exactly this. I often have small or large browser windows, and being able to choose instead of being constrained by fixed-width sites is great.
With the client side solutions, like responsive CSS, you're serving one cached, distributed, minified set of data to every client.
Also only sending the needed CSS for mobile browsers makes it that much faster.
(Resize your window and you'll see how everything re-aligns for desktops, tablets, and smartphones)
I chose responsive css because I can code it once, and it'll work on all devices. No bloat, no separate versions to maintain. Why wouldn't you want that? Especially with more and more consumers buying mobile and tablet devices. You increase your potential audience with a few lines of css code. Why do it through server-side code and have seperate versions to maintain during each and every update? As long as you plan ahead, your design can be responsive with literally a few lines of css code.