Responsive web design testing is the systematic verification that your website works properly across the full spectrum of devices and screen sizes your visitors use. You're checking that layouts reflow correctly, images scale without breaking, navigation remains usable, forms submit properly, and interactive elements respond to touch and click inputs appropriately. The goal is catching layout breaks, unusable buttons, horizontal scrolling issues, and functionality failures before real users encounter them. Testing happens on three levels. Device testing means physically checking the site on actual phones, tablets, and desktops—an iPhone 14, Samsung Galaxy, iPad, various Windows and Mac laptops. Browser testing covers Chrome, Safari, Firefox, Edge, and their mobile variants, since rendering engines handle CSS differently. Viewport testing uses browser dev tools or platforms like BrowserStack to simulate hundreds of screen widths from 320px phones up to 2560px monitors. The practical approach combines automation and manual checks. Automated tools like Lighthouse, WebPageTest, and Cypress can flag obvious breaks and performance issues across viewports. But you still need human eyes on key pages—homepage, product pages, checkout flows, contact forms—on real devices. A button that technically renders might still be too small to tap comfortably, something automation misses. At Ottawa SEO, we test client sites across at least six real devices we keep in-office plus emulated viewports for edge cases. We're looking for broken grids at 768px tablet breakpoints, images that don't load on slower 3G connections, and navigation menus that overlap content at specific widths. We also check that Canadian-specific elements like bilingual toggles and province dropdowns work smoothly on mobile. Most layout breaks happen at breakpoints (768px, 1024px, 1280px) where CSS media queries switch designs. Test those thresholds plus or minus 50px. Also test in both portrait and landscape orientations on tablets. The 15 minutes spent catching a broken mobile menu before launch saves dozens of support tickets and lost conversions after.