Most sites still get designed on a desktop monitor first, then adapted down for mobile. That order is backwards for most businesses now, and it shows up directly in conversion numbers.
Client review calls happen on laptops. Designs get approved looking good on a big screen, and mobile becomes an afterthought pass rather than the primary design target it should be for most consumer-facing sites.
Primary actions need to sit where a thumb naturally reaches, not just where they look balanced in a desktop mockup.
Fewer required fields, autofill support, and clear indication of progress. Every extra field costs more on mobile than desktop.
This is where most ecommerce conversion is lost or won. A checkout that requires precise taps on a cluttered screen loses customers mid-purchase.
Sites with genuinely mobile-first checkout and navigation consistently see measurable lifts in mobile conversion rate after a rebuild, often in the double digits, simply from removing friction that was invisible on desktop.
Load the site on an actual phone, not a resized browser window. Try to complete the primary action (purchase, signup, contact) one-handed. Count the taps and required fields. If it takes more effort than it should, so will your client's customers.
Navigation simplicity, checkout friction, and image load performance on mobile bandwidth, in that order. These three account for most of the conversion gap between a desktop-first site and a mobile-first one. (We cover platform-specific mobile considerations in our WordPress vs Shopify guide.)
