Mobile-first
Designing for the phone screen first, then expanding the layout for bigger screens.
What it is
Mobile-first means starting with the smallest screen and working up. Phones force you to decide what really matters; bigger screens then get extra room for secondary content.
It's the right default for consumer apps where most people arrive on a phone. Business tools used at a desk are often better designed desktop-first.
Tell your coding agent which you want. A mobile-first app still needs a good desktop layout, and the reverse.
How to ask for it
“Most of my customers use their phones.”
“Design this mobile-first: start from a 390px-wide phone layout with a bottom tab bar and large tap targets, then expand to a two-column layout with a left sidebar on screens wider than 1024px.”
Stating the fact leaves the approach open; "mobile-first" sets the starting point and the prompt says how it scales up.
You've seen this in
- IInstagram's web app
- UUber Eats
- MMost banking apps' web versions