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.