A CartoCSS developer is responsible for creating and customizing map styles using CartoCSS, a language similar to CSS used for map design. They work on Carto, a platform that provides GIS and web mapping tools for display in a web browser. The developer manipulates features such as colors, widths, labels, and zoom level displays to create visually compelling maps. They also troubleshoot and optimize map performance and ensure consistency across different devices and screen sizes. Additionally, they may collaborate with data analysts and developers to integrate maps into websites or apps, enhancing user interaction and data visualization.