Custom PostCSS Config

Install PostCSS and the plugins you plan to use:

npm install postcss postcss-import postcss-preset-env cssnano postcss-mixins

Baseline ships fallback plugins, but when you provide your own postcss.config.js, bring the plugins as explicit dependencies.

Add postcss.config.js at the project root:

import postcssImport from "postcss-import";
import postcssMixins from "postcss-mixins";
import postcssPresetEnv from "postcss-preset-env";
import cssnano from "cssnano";

const isProd = process.env.ELEVENTY_ENV === "production";

const plugins = [
  postcssImport(),
  postcssMixins(),
  postcssPresetEnv({ stage: 1 }),
];

if (isProd) {
  plugins.push(cssnano({ preset: "default" }));
}

/** @type {import('postcss-load-config').Config} */
export default {
  map: isProd ? false : { inline: true },
  plugins
};

Exercise the config in your CSS (imports and mixins):

src/assets/css/base.css:

:root {
  color-scheme: light;
}

body {
  font-family: system-ui, sans-serif;
  margin: 0;
  padding: 2rem;
  line-height: 1.5;
  color: #1f2937;
  background: #f8fafc;
}

h1 {
  margin-bottom: 0.5rem;
}

.pill {
  display: inline-block;
  padding: 0.25rem 0.75rem;
  border-radius: 999px;
  background: #e0f2fe;
  color: #0f172a;
  font-weight: 600;
}

src/assets/css/mixins.css:

@define-mixin linksMixin {
  color: #0f172a;
  text-decoration: underline;
  text-decoration-color: #fc35b9;
  text-decoration-thickness: 2px;
  text-underline-offset: 3px;
  font-weight: 600;
}

src/assets/css/links.css:

a {
  @mixin linksMixin;
}

a:hover {
 color: #fc35b9;
}

src/assets/css/index.css:

@import "./base.css";
@import "./mixins.css";
@import "./links.css";

Run and inspect.

• Dev: npx rimraf dist/ && npm run dev
• Build: npx rimraf dist/ && npm run build
• Check dist/assets/css/index.css:
  • Dev: expanded CSS with mapped imports; no minification.
  • Prod: minified (cssnano), imports flattened, custom media resolved.