What is mobile-first design?
Mobile-first design is a design approach in which the smallest screen size is designed before larger ones. Rather than designing for desktop and then adapting for mobile, mobile-first starts with the constraints of a small screen and limited input method, then progressively enhances the experience for larger screens and more capable devices. The approach was popularized by Luke Wroblewski and has become a standard practice in responsive web design, supported by Google's mobile-first indexing which uses the mobile version of a site for search ranking.
Why does mobile-first produce better design?
Mobile-first design forces designers to prioritize ruthlessly because small screens cannot accommodate everything. When a design must fit on a 375px wide screen, every element must justify its presence. Content that is not essential gets removed or deferred. Navigation that is unclear on desktop becomes obviously problematic when compressed to mobile. cognitive load that is tolerable on a large screen with a mouse becomes immediately apparent on a small touchscreen. The constraints of mobile act as a quality filter that often produces cleaner, more focused interfaces that also work well when scaled up to desktop.
What is the difference between mobile-first and responsive design?
Responsive design is a technical approach in which a single codebase adjusts its layout in response to the screen size of the device. Mobile-first is a design strategy that determines which screen size to design for first. The two concepts are related but independent: a responsive design can be built from desktop-first or mobile-first designs. Mobile-first as a design strategy pairs naturally with responsive implementation because starting with mobile constraints means the layout has a solid foundation to scale from rather than needing to be compressed down from a larger starting point.