Eliminating Unused CSS

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Frameworks like Bootstrap or Tailwind tend to come with a lot of CSS. Often you use only a small part of it and if you aren't careful, you will bundle even the unused CSS. There's tooling that exists to work around the problem.

PurgeCSS is a tool that can achieve this by analyzing files. It walks through your code and figures out which CSS classes are being used. Often there is enough information for it to strip unused CSS from your project. It also works with single page applications to an extent.

uncss is a good alternative to PurgeCSS. It operates through PhantomJS and performs its work differently. You can use uncss itself as a PostCSS plugin. dropcss is another option.

Given PurgeCSS works the best with webpack, we'll demonstrate the usage in this chapter.

Setting up Tailwind#

To make the demo more realistic, let's install Tailwind to the project.

npm add tailwindcss postcss-loader --develop

To load Tailwind, we'll have to use PostCSS:


exports.tailwind = () => ({
  loader: "postcss-loader",
  options: {
    postcssOptions: {
      plugins: [require("tailwindcss")()],

The new configuration still needs to be connected:


const cssLoaders = [parts.tailwind()];
const productionConfig = merge([parts.extractCSS()]);
const productionConfig = merge([ parts.extractCSS({ loaders: cssLoaders }), ]);
const developmentConfig = merge([ parts.devServer(),
parts.extractCSS({ options: { hmr: true } }),
parts.extractCSS({ options: { hmr: true }, loaders: cssLoaders }),

To make the project aware of Tailwind, import it from CSS:


@tailwind base;
@tailwind components;

/* Write your utility classes here */

@tailwind utilities;

body {
  background: cornsilk;

Using Tailwind classes#

You should also make the demo component use Tailwind classes, so there is something to work with:


export default (text = "Hello world") => {
  const element = document.createElement("div");

  element.className = "rounded bg-red-100 border max-w-md m-4 p-4";
  element.innerHTML = text;

  return element;

If you run the application (npm start), the "Hello world" should look like a button.

Styled hello
Styled hello

Building the application (npm run build) should yield output:

Hash: 6ed243a3e5aade0133d5
Version: webpack 4.43.0
Time: 1282ms
Built at: 07/09/2020 4:24:36 PM
     Asset       Size  Chunks                    Chunk Names
index.html  237 bytes          [emitted]
  main.css   1.32 MiB       0  [emitted]  [big]  main
   main.js   1.12 KiB       0  [emitted]         main
Entrypoint main [big] = main.css main.js
[0] ./src/main.css 39 bytes {0} [built]
[1] ./src/index.js + 1 modules 315 bytes {0} [built]
    | ./src/index.js 99 bytes [built]
    | ./src/component.js 211 bytes [built]
    + 1 hidden module

As you can see, the size of the CSS file grew, and this is something to fix with PurgeCSS.

Enabling PurgeCSS#

purgecss-webpack-plugin allows you to eliminate most of the CSS as ideally we would bundle only the CSS classes we are using.

npm add glob purgecss-webpack-plugin --develop

You also need to configure as below:


const path = require("path");
const glob = require("glob");
const PurgeCSSPlugin = require("purgecss-webpack-plugin");

const ALL_FILES = glob.sync(path.join(__dirname, "src/*.js"));

exports.eliminateUnusedCSS = () => ({
  plugins: [
    new PurgeCSSPlugin({
      paths: ALL_FILES, // Consider extracting as a parameter
      extractors: [
          extractor: (content) =>
            content.match(/[^<>"'`\s]*[^<>"'`\s:]/g) || [],
          extensions: ["html"],
For exceptions, PurgeCSS 3.0 includes safelist and blocklist options.

Next, the part has to be connected with the configuration. It's essential the plugin is used after the MiniCssExtractPlugin; otherwise, it doesn't work:


const productionConfig = merge([
  parts.extractCSS({ loaders: cssLoaders }),

The order of the CSS related calls doesn't matter as the plugins will register to different parts of the build.

If you execute npm run build now, you should see something:

Hash: 6ed243a3e5aade0133d5
Version: webpack 4.43.0
Time: 1494ms
Built at: 07/09/2020 4:35:27 PM
     Asset       Size  Chunks             Chunk Names
index.html  237 bytes          [emitted]
  main.css   7.28 KiB       0  [emitted]  main
   main.js   1.12 KiB       0  [emitted]  main

The size of the style has decreased noticeably. Instead of 1.32, you have roughly 7k now. The difference would be even more significant for more massive CSS frameworks.

Tailwind includes PurgeCSS out of the box and it can be preferable to use that. See Tailwind documentation for more information. The example above is enough to illustrate the idea, and it works universally.

Critical path rendering#

The idea of critical path rendering takes a look at CSS performance from a different angle. Instead of optimizing for size, it optimizes for render order and emphasizes above-the-fold CSS. The result is achieved by rendering the page and then figuring out which rules are required to obtain the shown result.

critical-path-css-tools by Addy Osmani lists tools related to the approach.


Using PurgeCSS can lead to a significant decrease in file size. It's mainly valuable for static sites that rely on a massive CSS framework. The more dynamic a site or an application becomes, the harder it becomes to analyze reliably.

To recap:

  • Eliminating unused CSS is possible using PurgeCSS. It performs static analysis against the source.
  • The functionality can be enabled through purgecss-webpack-plugin.
  • At best, PurgeCSS can eliminate most, if not all, unused CSS rules.
  • Critical path rendering is another CSS technique that emphasizes rendering the above-the-fold CSS first. The idea is to render something as fast as possible instead of waiting for all CSS to load.

In the next chapter, you'll learn to autoprefix. Enabling the feature makes it more convenient to develop complicated CSS setups that work with older browsers as well.

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