What is responsive web design?

Responsive web design (RWD) is an approach where a website's layout and content adapt to look good on any screen size, from mobile phones to large desktop monitors. The three core principles are: fluid grids (using percentages or fr units instead of fixed pixels), flexible images (max-width: 100% so images never overflow their containers), and media queries (changing styles at specific breakpoints). A mobile-first strategy writes base styles for mobile then uses min-width media queries to add complexity for larger screens. Responsive design replaced separate mobile sites and is now the industry standard approach.