Responsive web design rests on three core principles: fluid grids, flexible media, and media queries. Fluid grids use relative units like percentages or viewport widths instead of fixed pixels, so layout elements resize proportionally to the screen. Flexible images and videos constrain themselves to their containers using max-width: 100%, preventing overflow on small screens. Media queries detect viewport dimensions and apply different CSS rules at breakpoints, typically around 768px for tablets and 480px for phones, though modern approaches often use more granular breakpoints or container queries. Beyond the technical foundation, responsive design demands mobile-first thinking. Starting with the smallest screen forces you to prioritize content and functionality, then progressively enhance for larger displays. This approach naturally produces faster mobile experiences because you're not hiding or overriding desktop bloat. Touch targets need at least 44x44 pixels of tappable area. Navigation patterns shift from horizontal menus to hamburgers or bottom bars. Forms stack vertically and use appropriate input types to trigger the right mobile keyboards. Performance is inseparable from responsiveness. Serving a 2MB hero image to a phone on LTE defeats the purpose of a fluid layout. Use srcset and picture elements to deliver appropriately sized images, or implement lazy loading for below-fold content. Core Web Vitals now punish layout shifts, so reserve space for images and ads before they load. At Ottawa SEO, we test responsive builds across real devices, not just browser dev tools, because iOS Safari and Chrome mobile handle viewport units and touch events differently. We also audit competitors' mobile experiences in target markets like Ottawa, Toronto, and Montreal to identify UX patterns users already expect. A site that's technically responsive but requires constant pinch-zooming or hunting for CTAs still fails the user. The goal isn't just flexibility, it's usability across contexts: commuter on subway, exec on iPad, desktop researcher. Every breakpoint decision should serve that outcome, not just check a box.