A concise guide to responsive design that defines common CSS breakpoints—phones: xs 320–480, sm 481–768; tablets: md 769–1024, lg 1025–1440; desktops: xl 1441–1920—and shows how to use them. It promotes mobile-first thinking, custom breakpoints when needed, and a test‑iterate workflow, illustrated by a travel booking page example to achieve a seamless, user-friendly experience across devices.