A Modernizr.js developer uses the Modernizr JavaScript library to detect the features available in a user's browser. This helps in making responsive designs and progressive enhancements by identifying and features that the user’s browser can or cannot support. They write conditional JavaScript and CSS code to provide fallbacks for unsupported features, ensuring the website functions optimally across various browsers. Furthermore, they contribute to enhancing user experience, website performance, and cross-browser compatibility. They may also be involved in testing, debugging, and improving the website's performance based on the feature detections made by Modernizr.js.