Responsive web design works by combining three technical components: flexible grid layouts, fluid images, and CSS media queries. Instead of building separate sites for desktop and mobile, you create one codebase that adapts based on the device accessing it. The flexible grid uses relative units like percentages or viewport widths instead of fixed pixels. A container might be set to 100% width with max-width: 1200px, so it fills a phone screen completely but stays readable on a 27-inch monitor. Columns stack vertically on narrow screens and sit side-by-side on wider ones. CSS media queries detect the viewport dimensions and apply different style rules at specific breakpoints. Common breakpoints are 768px for tablets and 1024px or 1200px for desktops, though we often add custom ones based on where the design actually breaks. When the browser width drops below 768px, a media query might change a three-column grid to a single column and increase font sizes for readability. Fluid images scale proportionally using max-width: 100% and height: auto, preventing them from overflowing their containers or pixelating on high-resolution screens. Modern implementations also use srcset attributes to serve appropriately sized images based on device capabilities, saving bandwidth on mobile connections. The browser does the heavy lifting. When someone visits your site, their device reports its viewport size, pixel density, and capabilities. Your CSS cascades through the media queries, applying the rules that match. No server-side device detection needed. At Ottawa SEO, we build mobile-first, writing base styles for small screens then progressively enhancing for larger viewports. This approach ensures fast load times on slower mobile connections and forces us to prioritize content hierarchy. We test across actual devices, not just browser resize tools, because touch targets, font rendering, and performance vary significantly between an iPhone 13, a Samsung Galaxy, and a desktop Chrome window. The goal is functional design at every size, not pixel-perfect consistency across all of them.