A Modernizr developer utilizes the Modernizr JavaScript library to detect and features in various browsers. They identify inconsistencies in browser support and create fallbacks, ensuring that web applications function correctly across different platforms. Developers use Modernizr to build responsive designs, enabling smooth user experiences regardless of device or browser type. They also optimize website performance and ensure accessibility. Their work involves coding, testing, debugging, and troubleshooting. They work closely with other developers and designers, contributing to the overall functionality and aesthetic of a website or web application.