The document discusses responsive design, which allows websites to automatically adapt their layout to different screen sizes and devices like desktops, mobile phones, and tablets. It explains that responsive design uses CSS media queries to apply different styling based on screen width, providing custom styles for the same HTML content on different devices. While responsive design is a step forward, it has some browser compatibility issues and images may not resize responsively, though JavaScript can help address these limitations. Overall, responsive design allows a single website to be optimized for all devices.