Responsive web design requires three technical foundations working together: fluid grid layouts, flexible media, and CSS media queries. Use relative units like percentages or viewport widths (vw) for your grid containers instead of fixed pixel widths. Set images and videos to max-width: 100% and height: auto so they scale within their containers without breaking layouts. The mobile-first approach is standard practice now. Write your base CSS for the smallest screens (typically 320px), then use min-width media queries to layer on complexity as viewports expand. Common breakpoints are 768px for tablets and 1024px or 1200px for desktops, but test your actual content to find natural breaking points rather than blindly following device dimensions. CSS Flexbox and Grid have replaced most float-based layouts. Flexbox handles one-dimensional flows (navigation bars, card rows), while Grid excels at two-dimensional layouts (page structure, complex dashboards). Both respond naturally to viewport changes with minimal media query overhead. For navigation, consider the hamburger-to-horizontal pattern: stacked links on mobile that expand into a full menu bar above 768px. Use viewport meta tags in your HTML head: <meta name="viewport" content="width=device-width, initial-scale=1">. Without this, mobile browsers render at 980px and scale down, defeating your responsive intentions. Test on real devices or browser dev tools, but prioritize actual phones and tablets because emulators miss touch interactions and performance issues. At Ottawa SEO, we build responsive by default since 2014 because Google's mobile-first indexing makes it non-negotiable for SEO. Watch out for performance: responsive doesn't mean serving desktop-sized images to mobile users. Implement srcset and picture elements to serve appropriately sized assets, or use a CDN with automatic image optimization. The biggest mistake is designing for desktop first and trying to retrofit mobile as an afterthought. You end up with awkward breakpoints and bloated CSS. Start small, enhance upward, and your code stays cleaner and your sites load faster across all devices.