CSS (Cascading Style Sheets) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media. CSS is one of the core languages of the open web and is standardized across web browsers according to W3C specifications. It allows developers to separate content from presentation, making websites more maintainable and accessible. CSS uses a simple syntax with selectors to target HTML elements and properties to define their style, such as color, font, size, and layout. It supports various features like media queries for responsive design, animations, and transitions to create dynamic and visually appealing web experiences. CSS can be included directly in HTML documents, linked as external files, or embedded within style tags. External CSS files are the most common and recommended approach for larger projects, promoting code reusability and maintainability.