Responsive websites are those crafted to use W3C CSS3 media queries with fluid grids and commonly also fluid images, to adapt the layout to the viewing environment. The idea of responsive web design is to make websites that fit multiple screen sizes. For the designer and developer that means they only have to work on one version of the website instead of designing one for desktop computers and another for mobile devices.

Fluid grids, flexible images, and media queries are the three technical ingredients for responsive web design, but it also requires a different way of thinking.