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SASS (Syntactically Awesome Stylesheets) is a CSS pre-processor that lets you use variables, mathematical operations, mixins, loops, functions, imports, and other functionalities that make writing CSS much more powerful. It will take your preprocessed Sass file and save it as a normal CSS file that you can use on your website. SASS comes with two different syntaxes: SASS itself and SCSS, the most used one. You don't need to spend hours learning Saas, you can easily pick up the syntax as you go.