An developer is responsible for designing, developing, and modifying websites and applications using , a markup language for structifying and presenting content on the web. They work on the layout of a website, create and insert graphics, and ensure website compatibility across different browsers and devices. They also work closely with CSS and JavaScript, which handle the appearance and interactive elements of a site, respectively. Their role includes debugging and improving usability, implementing responsive design for mobile devices, and ensuring the website meets SEO standards. They often collaborate with other professionals like web designers and backend developers.