Starting mobile-first forces clarity: on a small screen you must prioritize the most important content and actions, which usually improves the desktop version too. Designing desktop-first often produces a cluttered phone experience that's hard to fix later.
In practice it pairs with responsive design: build the core mobile layout, then add complexity for wider screens. Testing on real phones, not just a resized browser, is what ensures the experience actually holds up where most of your visitors are.
Updated July 2026
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