Mobile-first design means building websites and apps with phones in mind first....

https://felixssuperperspective.brightsora.com/posts/why-does-my-website-look-fine-on-desktop-but-messy-on-my-phone

Mobile-first design means building websites and apps with phones in mind first. It makes everything load faster, so you don’t waste time waiting. Buttons and links are big enough to tap easily with your fingers