A solid foundation for building responsive sites the right way, starting at 320px and scaling up through clear breakpoints. You get sensible defaults like 48px touch targets, progressive enhancement patterns, and performance budgets that actually matter (under 3s FCP on 3G). The CSS examples show proper min-width media queries instead of the old max-width approach, and it pushes you toward semantic HTML that works before JavaScript loads. Nothing groundbreaking here, but it's the kind of practical reference that keeps you from making rookie mistakes like forgetting adequate spacing or shipping bloated bundles to mobile users.
npx skills add https://github.com/secondsky/claude-skills --skill mobile-first-design