Mobile-first design means building websites and apps to work great on phones...

https://eduardosnicechat.publishlane.com/posts/why-do-streaming-apps-change-what-people-expect-from-other-websites

Mobile-first design means building websites and apps to work great on phones first. It focuses on fast loading, easy-to-tap buttons, and simple menus that fit small screens. This makes browsing quick and smooth, even on the go