Behind the scenes, responsive websites use a new feature of CSS3 called media queries, to detect the screen size, and style the website differently according to the different sizes. Typically a designer will break content up into blocks that rearrange themselves at certain screen sizes. So the site will apply one set of CSS styles for screens above a certain size, and a different set of styles to viewports below that size, and so on. On smaller screens, content blocks will stack on top of each other for easy mobile viewing.