What is the mobile-first approach in responsive design?

Explanation

The mobile-first approach starts with base CSS styles targeting the smallest screen size (mobile), then uses min-width media queries to progressively add enhancements for larger screens. This approach aligns with progressive enhancement philosophy, ensures mobile performance (no unnecessary desktop styles loaded on mobile), and produces simpler CSS (additive rather than subtractive). It contrasts with desktop-first, which starts with desktop styles and uses max-width queries to remove features for smaller screens.