The first thing I want to say is in regards to design. You cannot have a responsive site without a responsive design. I recommend taking a “mobile first” approach because it forces you to focus on the most critical elements in your app. Once the most important elements are established, the rest can be deemphasized or taken away on smaller screens. This isn’t really a dev tip — for the rest of the article I’ll assume you have a polished design that must adapt to various screens.