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CSS preprocessor is a program that lets you generate CSS from the preprocessor’s own unique syntax. Preprocessors allow writing of regular CSS, just as you would write it anywhere else. The popular ones are Sass, Less, and Stylus. Sass is a way of saving reusable styles that can be used throughout your stylesheet. Sass allows nesting of selectors, and it improves the readability of the code. Using partials is a great way to modularize your code into smaller reusable parts.