preact.config.js

const notFoundError = name => `Please pass the ${name} parameter to plugin`;

const defaultParams = {
  regex: /[\w-/:]+(?<!:)/g,
};
// derived from https://github.com/agneym/preact-cli-tailwind
// I got an issue with font so I modify the source and also I added whitelist

module.exports = (config, env, helpers, params = defaultParams) => {
  if (!config) throw new Error(notFoundError("config"));
  if (!env) throw new Error(notFoundError("env"));
  if (!helpers) throw new Error(notFoundError("helpers"));
  
  const purgecss = require('@fullhuman/postcss-purgecss')({
    // Specify the paths to all of the template files in your project
    content: ['./build/**/*.js', './build/**/*.html'],
    css: ['./build/**/*.css'],
    whitelist: ['body'],
    defaultExtractor: content => content.match(/[\w-/:]+(?<!:)/g) || []
  })

  const postCssLoaders = helpers.getLoadersByName(config, "postcss-loader");
  postCssLoaders.forEach(({ loader }) => {
    const plugins = loader.options.plugins;

    // Add tailwind css at the top.
    plugins.unshift(require("tailwindcss"));

    // Add PurgeCSS only in production.
    if (env.production) {
      plugins.push(purgecss);
    }
  });
  return config;
};