Responsive web design works by building a single HTML codebase that reshapes itself based on the device viewing it. Instead of creating separate mobile and desktop sites, you use three core techniques: flexible grid layouts (percentages instead of fixed pixels), fluid images (max-width: 100% so they scale down), and CSS media queries that apply different styles at specific breakpoints. Media queries are the engine. When a browser loads your page, it checks the viewport width and executes CSS rules that match. For example, a three-column desktop layout might stack into a single column on phones using a @media (max-width: 768px) breakpoint. Common breakpoints sit around 480px (small phones), 768px (tablets), 1024px (small laptops), and 1200px+ (desktops), though we set them based on where the actual design breaks, not arbitrary device categories. The mobile-first approach starts with the smallest screen CSS and progressively enhances for larger viewports. This tends to produce leaner code and faster mobile load times, which matters since Google indexes the mobile version first. Desktop-first does the opposite but often ships unnecessary code to phones. Browser detection happens client-side. The HTML stays identical; only the applied CSS changes. JavaScript can add interactivity adjustments like touch-friendly navigation on mobile or hover effects on desktop. Flexbox and CSS Grid have largely replaced older float-based grids because they handle dynamic layouts more cleanly. At Ottawa SEO, we test responsive builds across real devices and Chrome DevTools viewports, checking that touch targets meet the 48px minimum, text remains readable without zooming, and critical content doesn't get buried in collapsed menus. Poor responsive implementation causes layout shifts that hurt Core Web Vitals and rankings. The goal isn't pixel-perfect replication across devices but ensuring usability and content accessibility everywhere. A properly responsive site eliminates duplicate content issues from separate mobile URLs and consolidates all ranking signals to one domain.