webpack-configs/static-html/postcss.config.js

const postcssFunctions = require('./src/js/postcss-functions');

module.exports = {
  plugins: [
    'postcss-import',
    'postcss-mixins',
    'postcss-preset-env',
    'postcss-nesting',
    'postcss-simple-vars',
    ['postcss-functions',
      {
        functions: postcssFunctions,
      },
    ],
  ],
};