This document provides an introduction to responsive web design. It defines responsive web design as crafting websites to provide an optimal experience across devices with screens. It distinguishes between adaptive and responsive design, with responsive design changing elements on the page based on device detection or browser width. The document outlines some basic and advanced ingredients for responsive design, including flexible grids, conditional images, media queries, touch input, and server-side conditionals. It emphasizes the importance of performance in responsive design and provides tips for optimizing websites for different networks and devices.