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CSS (Cascading Style Sheets) is not a programming language, it is the code you use to selectively style the web. CSS layout is based on the "box" model, with each *box* taking up space on the webpage having various properties like padding (which is the space around the content) and border (this is the solid line that encircles the padding), and margin. A CSS ruleset consists of the selector, declaration, property, and property value.