A Cascading Style Sheets (CSS) developer specializes in designing and implementing responsive and visually appealing web interfaces. They write CSS code to control the layout, colors, fonts, and animations of web pages. They also handle cross-browser compatibility issues, optimize web pages for maximum speed and scalability, and collaborate with other team members to define and apply style and user interface standards. Additionally, they may use CSS pre-processing platforms like SASS or LESS, work closely with HTML and JavaScript, and ensure the website's design is optimized for various devices. They also debug and fix issues in existing stylesheets.