Responsive web design means your website rearranges itself based on the device viewing it. A three-column desktop layout might stack into a single column on mobile, navigation menus collapse into hamburger icons, and images scale proportionally. This happens through three core techniques working together. First, flexible grid layouts use relative units like percentages or viewport widths instead of fixed pixels. A sidebar set to 33% width shrinks proportionally on smaller screens rather than breaking the layout. Second, fluid images are capped at 100% of their container width so they never overflow on narrow displays. Third, CSS media queries detect screen width and apply different styling rules at breakpoints, typically around 768px for tablets and 1024px for desktops, though we set these based on where the actual design breaks. The viewport meta tag in the HTML head is critical. Without it, mobile browsers render the page at desktop width and zoom out, defeating the entire purpose. You need <meta name="viewport" content="width=device-width, initial-scale=1"> to tell browsers to respect the device width. At Ottawa SEO, we build mobile-first, meaning we design for phones initially then progressively enhance for larger screens. This forces prioritization of essential content and typically results in faster load times. Google has used mobile-first indexing since 2019, so your mobile version is what primarily determines rankings. A non-responsive site risks lower search visibility and brutal bounce rates on mobile, which now accounts for 55-65% of web traffic in Canada. Common mistakes include setting breakpoints based on popular device dimensions rather than content needs, forgetting to test touch target sizes (buttons need 44x44px minimum), and ignoring performance since mobile users are often on slower connections. Responsive design is not optional anymore. It is the baseline expectation, and has been for a decade.