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

https://wiki-nest.win/index.php/What_Should_I_Fix_When_Users_Complain_%22The_Site_Is_Hard_to_Use_on_My_Phone%22%3F

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