Across service-business clients, mobile sessions make up 58–82% of organic traffic. We design every template at 360px first, then expand to tablet and desktop — not the other way around. The visible result of the opposite habit is sites where mobile heroes break, CTAs disappear under the fold, and forms scroll horizontally.
In practice: hero typography sized for mobile first, primary CTA at thumb height, sticky mobile bottom bar, image variants for at least three breakpoints, font subsetting, lazy-loaded below-the-fold media, and a hard ceiling on CLS measured on a mid-range Android over 4G.