This document introduces responsive web design (RWD). It discusses how early web design copied print and led to separate websites for different devices, which was expensive and impractical. RWD emerged to create designs that change based on screen size using CSS media queries. The key aspects of RWD are designing websites that respond appropriately to different devices through fluid, flexible layouts. It emphasizes designing for all possible screen widths from the start using relative units and media queries. Testing in the browser across screen sizes is important.