Mobile-first CSS layouts using Flexbox, Grid, media queries, and responsive images across all devices.

• Covers mobile-first design progression from 320px through large screens (1440px+) with standard breakpoint definitions
• Implements Flexbox for 1-dimensional layouts and CSS Grid for 2-dimensional layouts, with practical examples for navigation, cards, and dashboards
• Includes responsive image techniques: srcset for resolution-aware loading, picture element for art direction, and CSS background image switching
• Provides fluid typography using clamp() for viewport-relative sizing and container queries for component-level responsive behavior
• Requires viewport meta tag and enforces relative units (rem, em, %, vw/vh) with max-width constraints instead of fixed dimensions