An HTML and CSS developer designs and creates websites or web applications. HTML (Hyper Text Markup Language) is used for creating the structure of web pages, including text, images, and links. CSS (Cascading Style Sheets) is used for styling and layout of web pages such as colors, fonts, and design elements. The developer ensures websites are responsive, user-friendly, and visually appealing. They may also work on enhancing website functionality, improving site performance, and ensuring website compatibility across browsers and devices. Furthermore, they often collaborate with other professionals like graphic designers or back-end developers.