This document discusses responsive web design using CSS3 and HTML5. It defines responsive web design as a process that creates a single website that adapts to different screen sizes, devices, and orientations. The key benefits of responsive design are that it saves money and time, improves performance and SEO, and provides wider browser support. Responsive design works by using a flexible grid, scaling images and media, and CSS3 media queries to apply different styles based on screen capabilities. Popular frameworks that support responsive design are also mentioned.