Responsive web design adapts your site's layout to any screen size using three core techniques: fluid grids based on percentages rather than fixed pixels, flexible images that scale within their containers, and CSS media queries that apply different styles at specific breakpoints. The goal is one codebase that works everywhere instead of separate mobile and desktop versions. Start mobile-first. Design for the smallest screen (typically 320px wide) first, then progressively enhance for tablets and desktops. This forces you to prioritize content and keeps your CSS leaner. Set the viewport meta tag in your HTML head: <meta name="viewport" content="width=device-width, initial-scale=1">. Without this, mobile browsers render your site at desktop width and scale it down. Use relative units like percentages, em, or rem for widths and font sizes. A container set to width: 90% or max-width: 1200px will flex naturally. For images, set max-width: 100% and height: auto so they never overflow their parent. CSS Grid and Flexbox handle complex layouts that reflow cleanly at different sizes. Define breakpoints where your design needs to change. Common ones are 768px for tablets and 1200px for desktops, but test your actual content to see where it breaks. Media queries let you adjust typography, hide or show navigation elements, or switch from single-column to multi-column layouts. Test on real devices, not just browser DevTools. Chrome's responsive mode is useful but doesn't catch touch target sizes, actual font rendering, or performance on slower connections. At Ottawa SEO, we verify layouts on iPhones, Android phones, iPads, and desktop monitors before launch because clients often discover issues on their own devices first. Watch out for performance. Responsive doesn't mean loading a 3MB hero image on mobile. Use srcset for responsive images or CSS background-image with media queries to serve appropriately sized assets. A responsive layout that takes eight seconds to load on 4G defeats the purpose. Aim for under three seconds on mobile networks.