Snowpack

Sass

// snowpack.config.json
"plugins": ["@snowpack/plugin-sass"]

Sass is a stylesheet language that’s compiled to CSS. It allows you to use variables, nested rules, mixins, functions, and more, all with a fully CSS-compatible syntax. Sass helps keep large stylesheets well-organized and makes it easy to share design within and across projects.

To use Sass + PostCSS, check out this guide.