inline-css-html-webpack-plugin.js

const Entities = require('html-entities').AllHtmlEntities;

const htmlEntities = new Entities();

class InlineCssHtmlWebpackPlugin {
  apply(compiler) {
    compiler.plugin('compilation', compilation => {
      compilation.plugin('html-webpack-plugin-before-html-processing', (htmlPluginData, callback) => {
        for (let filename of htmlPluginData.assets.css) {
          const cssSrc = compilation.assets[filename];

          htmlPluginData.html = htmlPluginData.html
          .replace('</head>', `<style>${htmlEntities.encode(cssSrc.source())}</style></head>`);

          delete compilation.assets[filename];
        }

        htmlPluginData.assets.css = [];

        callback(null, htmlPluginData);
      });
    });
  }
}

module.exports = InlineCssHtmlWebpackPlugin;