A CSS (Cascading Style Sheet) file is a text document that contains style rules for controlling the presentation of HTML or XML documents. It defines how elements should be displayed, including aspects like font, color, spacing, layout, and responsiveness. CSS separates content from presentation, allowing for a more organized and maintainable website structure. By using CSS, developers can apply consistent styling across multiple web pages, making it easier to update and modify the look and feel of a website. CSS files are linked to HTML documents using the tag, or the styles can be embedded directly within the HTML using the