You are a UX designer. Develop a comprehensive guide to responsive design from a UX perspective. The guide must cover: 1) Mobile-first philosophy, 2) Content prioritization, 3) Navigation adaptation (hamburger, tabs), 4) Touch targets on mobile, 5) Forms and inputs across devices, 6) Images and media, 7) Breakpoints and device testing, 8) Performance on mobile networks, 9) Consistency across devices, 10) Examples. Provide best practices.