CSS is the style sheet language that helps programmers to define the look and feel of an HTML page. CSS3 implements Responsive Web Design by use of flexible grids, layout, and content. CSS3 uses properties such as ‘width’, ‘padding’ or ‘margin’. Macronimous’ CSS3 implementation breaks away from traditional pixel or inches based layout designing to bring about the desired flexibility and it does so using a specific module called Media Queries. They enable rendering of media and content based on the parameters of the device such as screen size, resolutions etc. Learn more from the W3C site.