Responsive design principles: mobile-first (start with mobile layout, add complexity for larger screens), fluid grids (use % or fr instead of fixed pixels), flexible images (max-width: 100%), media queries (@media (min-width: 768px)), CSS Grid and Flexbox for layout, relative units (rem, em, vw, vh). Common breakpoints: 640px (mobile), 768px (tablet), 1024px (laptop), 1280px (desktop). Test on real devices, not just browser resize.
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Responsive Design Guide
Build websites that work on every screen size.
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