Responsive web design is a development approach where a website automatically adjusts its layout, images, and navigation to fit any screen size, from smartphones to large desktop monitors. With mobile devices accounting for over 60% of global web traffic, a non-responsive site risks losing the majority of its visitors. Google also uses mobile-friendliness as a ranking factor, meaning a responsive design directly impacts your search engine visibility. Modern responsive techniques like CSS Grid, Flexbox, and container queries allow precise control over how content reflows, ensuring a smooth experience across hundreds of device variations.