A JavaServer Faces (JSF) developer designs and builds web applications using the JSF framework, a Java standard for building user interfaces in web applications. Their responsibilities include writing managed beans, developing custom UI components, handling events and server-side validation, and managing application state and navigation. They also implement AJAX calls, ensuring seamless user experiences without full page refreshes. Furthermore, they work with Expression Language to bind frontend components to backend data sources and use Facelets for templating. A JSF developer also needs to ensure the application's compatibility with various browsers and devices.