Mobile-first design means building a website or app starting with the smallest...

https://www.bookmarking-planet.win/mobile-first-design-means-building-your-website-or-app-starting-with-phones-in

Mobile-first design means building a website or app starting with the smallest screen—your phone. It focuses on speed, so pages load quickly, and uses big, easy-to-tap buttons for smooth navigation with your fingers