Mobile-first design means creating apps and websites that work great on phones...

https://wiki-cable.win/index.php/How_to_Design_Mobile_Pages_for_People_Who_Are_Multitasking

Mobile-first design means creating apps and websites that work great on phones first. It’s faster to load, with big buttons and easy-to-tap features that fit your screen. The menus are simple, so you can find what you need without fuss