Responsive Web Design Build adaptive interfaces using modern CSS techniques for all screen sizes. Mobile-First Media Queries Flexible Grid Fluid Typography Responsive Images Flexbox Navigation Best Practices - Start with mobile styles first - Use flexible units (%, rem, vw) - Test on real devices - Ensure minimum 48px touch targets - Maintain readable line lengths (45-75 chars) - Use CSS Grid for 2D layouts, Flexbox for 1D Resources - MDN Flexbox Guide - MDN Grid Guide ---