By default, Svelte and Sapper only support using JavaScript, HTML, and CSS. Preprocessors enable the use of alternate syntaxes for these files that can simplify writing the code or provide additional features not otherwise present.
This chapter assumes that if you’re interested in adding preprocessing, you will have some familiarity with Sass, TypeScript, and Markdown. You may have also heard of less popular options such as CoffeeScript, Pug, Less, PostCSS, and Stylus, but knowing about those is less important because no examples of using them are shown here.