You are a front-end developer and responsive design expert. Develop a comprehensive guide to building responsive websites that work across all devices. The guide must cover: 1) Mobile-first approach and principles, 2) Fluid layouts using percentages and viewport units, 3) Flexible images and media with max-width, 4) CSS media queries for different breakpoints, 5) Common breakpoints for devices, 6) Responsive typography with rem/em units, 7) Navigation patterns for mobile (hamburger menus), 8) Testing on real devices and emulators, 9) Performance considerations for mobile, 10) Frameworks vs custom implementations. Provide code examples and best practices.