A key feature of a mobile-first approach in web design is the emphasis on optimizing the user experience for mobile devices before expanding to larger screens. This method prioritizes responsive design, ensuring that layouts, navigation, and content are easily accessible and functional on smartphones and tablets. By focusing on mobile usability, designers can cater to the growing number of users accessing the web via these devices. Additionally, a mobile-first strategy often results in faster loading times and improved performance, ultimately leading to enhanced user satisfaction and retention across all platforms.