The programming language known for its use in web development alongside HTML and CSS is JavaScript. JavaScript is essential for adding interactive features and dynamic content to websites. It enables developers to create responsive user interfaces, manipulate the DOM (Document Object Model), and implement asynchronous communication through AJAX (Asynchronous JavaScript and XML). Furthermore, JavaScript supports various frameworks and libraries, such as React, Angular, and Vue.js, which streamline the development process. Its versatility allows it to run on both client-side and server-side environments, enhancing web application functionality.