Responsive design is about more than just layout; it’s about designing for the Web, which means, mostly, for people with browsers. And that’s just about everything we know about the people who visit our websites: they are probably using a browser. All the rest we just don’t know.
Up until not so long ago, we used to base our designs on some rather general assumptions about screen size and input type. With the rise of devices with various screen sizes and alternative ways to interact, these assumptions have turned out to be unreliable. We need to upgrade the defaults that we use when we start designing our websites.