Mobile-first is not a slogan, it is where your traffic is

Everyone says mobile-first. Most sites are still designed at 1440 pixels on a large monitor and then squeezed. The difference shows up in the numbers.

Design at the small size first

Starting narrow forces the hierarchy question immediately: what must be visible, and in what order. A layout that works at 375 pixels almost always expands gracefully. The reverse is not true. A desktop layout squeezed down becomes a long list of everything.

The specifics that matter

• Tap targets at least 44 pixels. Fingers are not cursors. Links in a row of text are the usual offender.
• Body text at 16px minimum. Anything smaller triggers zoom on iOS and looks like a mistake.
• No horizontal scrolling. Ever. Usually caused by a fixed-width image or table. Check at 320px, the narrowest device still in real use.
• Thumb reach. Primary actions in the lower two-thirds of the screen, not tucked in a top corner.
• Form inputs with the right type. type="email", type="tel", inputmode="numeric". It changes the keyboard and it removes real friction.
• Sticky elements sparingly. A header plus a cookie bar plus a chat bubble can leave a third of a small screen for content.

Performance is a mobile problem

Your visitor is on a three-year-old phone on a train. Every unnecessary script is felt. Test with throttling on, not on office wifi, and look at Core Web Vitals field data rather than a lab score. That is what Google uses and what your visitor experiences.

The test nobody does

Complete your own most important task, such as buying something or sending the enquiry, on your phone, outside, one-handed, in sunlight. Nearly every mobile problem a site has is found in that one attempt, and none of it is visible in a browser's device emulator.

Where desktop still matters

B2B research often happens on a laptop even when the first visit was mobile, and long forms are still completed on desktop. Mobile-first does not mean desktop-last; it means deciding priorities where space is scarce, then using the extra room deliberately.