The foundational language of the web alongside HTML (HyperText Markup Language) is CSS (Cascading Style Sheets). CSS is used to control the presentation and layout of web pages, allowing developers to separate content from design. This enables improved accessibility and maintains a more organized code structure. With CSS, developers can apply styles such as fonts, colors, and spacing consistently across multiple pages, enhancing user experience. Furthermore, CSS is essential for responsive web design, making websites adaptable to different screen sizes and devices.