Compile CSS with Parcel CSS

.eleventy.js

const css = require("@parcel/css");

module.exports = (eleventyConfig) => {
  eleventyConfig.addTemplateFormats("css");

  eleventyConfig.addExtension("css", {
    outputFileExtension: "css",
    compile: (contents, inputPath) => {
      return (data) => {
        let ret = css.bundle({
          filename: inputPath,
          minify: true,
        });
        return ret.code;
      };
    },
  });

  //...
};