Responsive design requires three technical foundations working together. First, you set a viewport meta tag in your HTML head so mobile browsers render at device width instead of desktop width. Second, you build layouts using relative units—percentages, rems, or CSS Grid fractions—instead of fixed pixels. Third, you write CSS media queries that trigger layout changes at specific breakpoints, typically 640px (mobile), 768px (tablet), 1024px (desktop), and 1280px+ (wide screens). The mobile-first approach means you write base styles for small screens, then layer on complexity for larger viewports using min-width media queries. This keeps your CSS lighter and forces you to prioritize content hierarchy. For images, you use max-width: 100% and height: auto so they shrink to fit their containers, or you serve different image sizes using srcset attributes to save bandwidth on mobile. At Ottawa SEO, we typically use Tailwind CSS for client projects because its responsive utility classes let us prototype breakpoints directly in markup without writing custom media queries for every component. A Tailwind class like "grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3" handles mobile stacking and desktop columns in one line. For WordPress builds, we customize Gutenberg blocks to respect the same breakpoints so clients don't break layouts when editing. The biggest implementation mistakes are testing only on your laptop's resized browser window instead of real devices, forgetting touch target sizes (Apple recommends 44px minimum), and ignoring landscape orientation on tablets. Performance matters more on mobile networks, so responsive implementation also means lazy-loading images below the fold and reducing JavaScript payloads. We use Chrome DevTools device emulation during development but always validate on actual iPhones and Android devices before launch because rendering engines behave differently than emulators suggest. Responsive isn't just about width—it's about input method, network speed, and context of use.