Direct Answer
Mobile-first design starts with the smallest screen and progressively layers in tablet and desktop styles. It matters because 64% of small-business website traffic in the U.S. comes from mobile (Statista 2024) and Google indexes the mobile version of every site by default through mobile-first indexing.
Voice answer (≤30 words)
“Mobile-first design builds the small-screen layout first. It matters because Google indexes the mobile version of your site by default.”
Mobile-first sites have lower CSS payload because they avoid unused desktop styles, which directly improves Core Web Vitals scores on slow connections.
Design at three breakpoints minimum: 375px (mobile), 768px (tablet), 1280px (desktop). Test on real devices, not just browser dev tools.
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