Responsive design boils down to three technical foundations: fluid grids that scale proportionally instead of fixed pixels, images that resize within their containers, and media queries that apply different CSS rules at specific viewport widths. You build one codebase that adapts rather than maintaining separate mobile and desktop sites. Start mobile-first. Write your base CSS for the smallest screens (320px-ish), then use min-width media queries to layer in complexity as viewports grow. Common breakpoints sit around 640px (tablets), 1024px (small laptops), and 1280px (desktops), but test your actual content to find where it breaks rather than cargo-culting standard numbers. A navigation that works at 639px but fails at 640px means your breakpoint is wrong. Use modern layout tools. CSS Grid handles two-dimensional layouts (rows and columns together), Flexbox manages one-dimensional flows (navigation bars, card rows). Both beat floats and positioning hacks by miles. Set images and videos to max-width: 100% and height: auto so they never overflow their containers. For complex images, use the picture element with multiple sources or srcset to serve appropriate file sizes. Test on real devices, not just browser resize. An iPhone SE, iPad, and MacBook show you actual rendering quirks that Chrome DevTools won't catch. Pay attention to touch targets (minimum 44x44px), font scaling (16px minimum for body text to avoid iOS zoom), and how forms behave with on-screen keyboards. At Ottawa SEO, we use Tailwind CSS for most builds because its utility classes make responsive adjustments explicit in the HTML (md:grid-cols-3, lg:text-xl). This prevents the responsive logic from getting buried in sprawling stylesheet files. We also run Lighthouse mobile audits on every page, aiming for 90+ performance scores, since Google's mobile-first indexing means your mobile version is your real version. The sites that rank aren't just responsive in theory—they load fast and work perfectly on actual phones in actual Canadian networks.