The document discusses responsive web design (RWD), which allows websites to automatically adapt to different screen sizes through flexible grids and images, and media queries. RWD uses fluid layouts where elements are sized using relative units like percentages. It also leverages CSS3 media queries to apply different styling for various devices. The document provides an overview of RWD and examples of how to structure HTML and CSS for a responsive design that can accommodate diverse screen sizes. It also lists resources for learning more about this approach to building adaptive and mobile-friendly websites.