Responsive web design is a front-end development approach where your HTML structure and CSS styling work together to create layouts that fluidly adjust to any screen size—from a 320px phone to a 2560px monitor. You're writing one HTML document that serves all devices, then using CSS techniques to rearrange, resize, or hide elements based on viewport width. The core mechanics are three: fluid grids using percentage-based widths or CSS Grid/Flexbox instead of fixed pixels, flexible images that scale with their containers (typically max-width: 100% and height: auto in CSS), and media queries that apply different CSS rules at specific breakpoints. A common breakpoint setup might be 768px for tablets and 1024px or 1200px for desktops, though we increasingly design mobile-first and add complexity upward. In practice, you'll structure your HTML semantically—header, nav, main, aside, footer—without prescribing exact positions. Then your CSS handles presentation. For example, a three-column desktop layout might stack into a single column on mobile using flexbox with flex-direction: column inside a media query for screens under 768px. Navigation often transforms from a horizontal bar to a hamburger menu using display properties toggled by breakpoint. The viewport meta tag in your HTML head is non-negotiable: <meta name="viewport" content="width=device-width, initial-scale=1">. Without it, mobile browsers render at 980px and scale down, breaking your responsive layout. At Ottawa SEO, we see responsive design as baseline—Google's been mobile-first indexing since 2019, meaning your mobile experience directly affects rankings. We typically build with CSS Grid for page-level layouts and Flexbox for component-level arrangements, test across real devices (not just browser dev tools), and optimize for Core Web Vitals since layout shifts on resize hurt both UX and SEO. The alternative—separate mobile subdomains or dynamic serving—creates maintenance overhead and canonicalization headaches that most businesses don't need.