In the past, a developer or designer might code a different site for desktop and mobile users, often with some or all of the same functionality. For the most part, in fact, this is still how things are done today. But in some cases, that's beginning to change. A relatively new technique called "responsive design" has been gaining traction over the past few years, and it's promising to change the way we code and interact with the Internet on devices of all shapes and sizes.