CSS At-rules are special instructions that provide additional information to the browser about how to interpret and render a web page. They are used to define the behavior of a document, such as setting the page size, specifying the font to use, or controlling the display of elements on the page.
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Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language, such as HTML or XML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.