A major characteristic of mobile-first design in web development is that it prioritizes the user experience on mobile devices over desktop versions. This approach encourages designers to create websites starting with smaller screens, ensuring that essential functions and content are accessible and user-friendly. By focusing on mobile optimization, developers can ensure faster load times, intuitive navigation, and responsive layouts. This is crucial, given the increase in mobile device usage. A mobile-first strategy not only improves user engagement and retention but also enhances search engine rankings due to mobile-friendly criteria set by platforms like Google.