When you're building out a responsive web design, attempting to provide a single design that works everywhere, on every device, it's a simple fact that you'll frequently encounter situations when features or techniques are not supported on certain devices. In these instances you'll likely want to create a fork in your CSS; if the browser supports a feature, provide one chunk of code, if they don't, they get different code. It's the kind of situation that gets handled by if/else or switch statements in JavaScript.

We currently have two possible approaches. One is entirely CSS based but with fewer browser implementations, and the other is only made possible with the help of a JavaScript library but enjoys far broader support. Let's consider each in turn.