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Mobile-first means starting your design from the smallest screen you want to support and then incrementally adding features/elements to it as you increase the screen size. Starting in 2019 Google now crawls and indexes new websites looking at their mobile version first. Mobile web traffic growth has skyrocketed since the massification of smartphones, by 2016 it overtook the desktop for the first time. The bottom-up approach of mobile-first will not only deliver great mobile experiences that let you reach bigger audiences but it will also keep you focused on what’s truly important.