Responsive web design officially started on May 25, 2010, when designer Ethan Marcotte published "Responsive Web Design" in A List Apart. That article introduced the term and outlined three core techniques: fluid grids based on percentages instead of fixed pixels, flexible images that scale within containers, and CSS3 media queries to apply different styles at different viewport widths. Before 2010, developers built separate mobile sites (m.domain.com) or ignored mobile entirely, which created maintenance nightmares and fragmented user experiences. The timing wasn't random. The iPhone launched in 2007, Android gained traction by 2009, and iPad arrived in early 2010. Suddenly, traffic from wildly different screen sizes became unavoidable. Marcotte's framework gave developers a practical alternative to device detection and duplicate codebases. By 2012, responsive design became the industry standard, reinforced when Google started penalizing non-mobile-friendly sites in 2015. The technical foundation existed earlier. CSS media queries appeared in the CSS3 spec around 2001, but browser support lagged until 2009-2010 when Firefox, Chrome, and Safari implemented them. Fluid grids date back to the early web, but designers mostly abandoned them for pixel-perfect Photoshop comps during the 2000s. Marcotte's contribution was synthesizing existing tools into a cohesive methodology at the exact moment the market needed it. At Ottawa SEO, we consider 2010 the hard start date for practical responsive design, but the real adoption curve ran 2011-2015. Sites we audit from that era often show hybrid approaches: desktop-first CSS with bolted-on media queries that break at odd widths. Modern responsive work flipped to mobile-first around 2013-2014, writing base styles for small screens and layering complexity upward. If you're evaluating an older site, anything pre-2015 likely needs a ground-up responsive rebuild rather than patches.