Responsive web design in CSS means your stylesheet adjusts layout, typography, and images based on viewport width. The core mechanism is media queries—CSS rules that apply only when certain conditions are met, typically screen width breakpoints like 768px for tablets or 1024px for desktops. Instead of fixed pixel widths, you use percentages, viewport units (vw, vh), or max-width declarations so elements scale proportionally. A typical responsive setup starts mobile-first: you write base styles for small screens, then layer in media queries for larger breakpoints. For example, a single-column layout on phones becomes two or three columns on desktop using flexbox or CSS Grid with different grid-template-columns values inside @media rules. Font sizes might shift from 16px to 18px or use clamp() functions to scale smoothly. Images need max-width: 100% and height: auto to prevent overflow. Key CSS techniques include: - Flexible grids using percentages or fr units in CSS Grid - Media queries targeting min-width or max-width breakpoints - Fluid typography with rem units or clamp() - Flexible images and videos with max-width constraints - Container queries (newer) for component-level responsiveness Common breakpoints we use at Ottawa SEO are 640px, 768px, 1024px, and 1280px, but these vary by project. Mobile traffic in Canada often exceeds 60% for local businesses, so mobile layout quality directly impacts conversions and Core Web Vitals scores. A non-responsive site gets penalized in mobile search rankings and frustrates users who pinch-zoom or scroll sideways. The alternative—building separate mobile and desktop sites—doubles maintenance cost and splits SEO authority. Responsive CSS consolidates everything into one codebase. Testing across real devices matters more than simulators because rendering engines handle flex wrap and media queries differently. Tools like Chrome DevTools device mode catch most issues, but always verify on actual iPhones and Android devices before launch.