A Markup Languages developer designs and creates web content using languages like HTML, XML, and XHTML. They structure and store data, define data types and their relationships, and ensure data is correctly displayed. They are responsible for the layout, visual appearance, and usability of a website, making it both user-friendly and effective. They work closely with web designers and back-end developers to implement their designs and ensure the website functions properly. They also test and troubleshoot issues, ensure web standards and accessibility guidelines are met, and may also assist in SEO strategy implementation.