Minifying a Custom Element that contains inline CSS and HTML

I've created a ShareButton web component, it's not great, but I like it and I've learned a lot about the Web Component ecosystem whilst I was creating it.

My main goal with this project was to encapsulate everything inside a single ES6 Class. I did this because I didn't want any non-developer controlled web request emanating from my element. I think I achieved this, but it came at the expense of some readability of the code because I had to inline the HTML and CSS that define the element.

It looked a lot like this:

const overlay = document.createElement('div'); = 'overlay';
overlay.innerHTML = `
  <div id="urlbar">
    <input type="url" id="url" />
    <button id="copy" aria-label="Copy to clipboard"><slot name="clipboard"><svg xmlns="" width="24" height="24"><path fill="none" d="M0 0h24v24H0z"/><path d="M16 1H4C3 1 2 2 2 3v14h2V3h12V1zm3 4H8C7 5 6 6 6 7v14c0 1 1 2 2 2h11c1 0 2-1 2-2V7c0-1-1-2-2-2zm0 16H8V7h11v14z"/></svg></slot></button>
    <button id="mailto" aria-lable="Mail to"><slot name="mailto"><svg xmlns="" width="24" height="24"><path fill="none" d="M0 0h24v24H0z"/><path d="M20 4H4c-1.1 0-1.99.9-1.99 2L2 18c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V6c0-1.1-.9-2-2-2zm0 14H4V8l8 5 8-5v10zm-8-7L4 6h16l-8 5z"/></svg></slot></button>
    <button id="android" aria-label="Share on Android"><slot name="android"><svg xmlns="" width="24" height="24"><path fill="none" d="M0 0h24v24H0z"/><path d="M6 18c0 .6.5 1 1 1h1v3.5c0 .8.7 1.5 1.5 1.5s1.5-.7 1.5-1.5V19h2v3.5c0 .8.7 1.5 1.5 1.5s1.5-.7 1.5-1.5V19h1c.6 0 1-.5 1-1V8H6v10zM3.5 8C2.7 8 2 8.7 2 9.5v7c0 .8.7 1.5 1.5 1.5S5 17.3 5 16.5v-7C5 8.7 4.3 8 3.5 8zm17 0c-.8 0-1.5.7-1.5 1.5v7c0 .8.7 1.5 1.5 1.5s1.5-.7 1.5-1.5v-7c0-.8-.7-1.5-1.5-1.5zm-5-5.8L16.8 1c.2-.3.2-.6 0-.8-.2-.2-.5-.2-.7 0l-1.3 1.4C13.7 1.2 13 1 12 1c-1 0-2 .2-2.7.6L7.8 0H7v1l1.5 1C7 3.2 6 5 6 7h12c0-2-1-3.8-2.5-4.8zM10 5H9V4h1v1zm5 0h-1V4h1v1z"/></svg></slot></button>
  <div class="buttons">
    <slot name="buttons"></slot>

It's not bad, but it also meant that that my element could not be minified to the best extent that it could be. In fact, outside of Polymer tooling (that focuses on parsing an HTML import), I could not find any tools or guidance about deploying a Custom Element that is optimized (let me know if I am mistaken).

I don't believe that as an element author I should force an entire build chain and tooling ecosystem on to the users of my element. I also believe that I should be able to give my users the smallest possible built file that offers users a fast by default experience without them having to jump through hoops.

I didn't want any super complex infrastructure that merges separate CSS and JS files into one output; I just wanted to have my development build of the custom element to work exactly the same as my deployed build... just smaller.

I hope with this article that at least you can see my process and understanding into how I minified my custom element.

Compressing ES6 class

I used babili. It's a very neat tool that compresses ES6 classes. It's still experimental, but I didn't run into any major issues.

npm install babili --save-dev

You can run it in a number of different ways, I chose to pipe data into it via STDOUT.

cat xyz | babili

This worked well, but it leaves the strings alone because it is assuming (rightly) that the output and formatting of those strings are intentional. If I want to get the element to be as small as possible, then I need to minify the CSS and HTML that are embedded in my element.

So how do you fix a problem like this?

I chose to break the problems of compressing the HTML and compressing the CSS into two extra steps.

Compressing inline CSS

I couldn't easily work out a scalable way to infer that a string was CSS and would require compressing, so I went old school and copied something that I had seen done in the Google Web Starter Kit - custom markers that define that something needs to be processed.

The custom marker would just be a normal comment that is ignored by the browser but a minifier would also strip out.

I wrote this simple script that reads from STDIN, loads the entire file, and then looks for a custom comment and then uses a simple regex to find the comment and the content in between before running a replacement function before finally printing the entire output back to STDOUT.

It includes clean-css, which seemed like it would do the trick.

const CleanCSS = require('clean-css');
const process = require('process');

let input = '';

process.stdin.on('readable', () => {
  let chunk =;
  if(chunk != null) input += chunk;

process.stdin.on('end', () => {
  let cssRe = /\/\*compress\*\/(.|[\n\r])+?\/\*endcompress\*\//;

  let compressedOutput = input.replace(cssRe, (match, p1, offset, string) => {
    let output = new CleanCSS({}).minify(match);
    return output.styles; 

I had to enhance my inline CSS with /*compress*/ and /*endcompress*/ comments, which might be a tad confusing for any developer reading the source but it does give me a lot of flexibility.

Compressing inline HTML

I did the exact same thing for the HTML minification. The html-minifier package is pretty neat, and I played around with it to find some sane defaults that worked well.

const minify = require('html-minifier').minify;
const process = require('process');

let input = '';

process.stdin.on('readable', () => {
  let chunk =;
  if(chunk != null) input += chunk;

process.stdin.on('end', () => {
  let cssRe = /<!--compress-->(.|[\n\r])+?<!--endcompress-->/;

  let compressedOutput = input.replace(cssRe, (match, p1, offset, string) => {
    return minify(match, { removeAttributeQuotes: true, removeComments: true, collapseWhitespace: true });

Hooking it all together

The share button has a custom build script that compresses the CSS, then the HTML, and finally it does the ES6 minification and ensures that the file intended for distribution and inclusion is as small as possible.

  "scripts": {
    "build": "cat share-button.js | node build/compress-css.js | node build/compress-html.js | babili > dist/share-button.min.js"

Integrating it in to my blog

I had intended to write a rant about deploying web components, but it was pretty simple in the end.

  1. Install the element from within my project:
npm install sharebutton --save
  1. Add it into my build deployment:
- npm install
- cp node_modules/sharebutton/dist/share-button.min.js static/javascripts/share-button.js

What's next?

I think it was worth it. The element is now 7917 bytes vs. 11700 bytes, which is roughly 33% smaller (gzipped, it's 2792 bytes vs. 3642), so I am happy for now (although I think I can do better). I also now have a process that I can use across my other elements, and everything is encapsulated into this one package.

I backed myself into a corner with my choice of inlining the CSS and HTML, but I still think it is the best solution for building custom elements.

The solution presented here will do a good job at minifying the element, but it will not do a great job, for example, because the custom element is opaque from the outside elements I could do a better job at minifying CSS selectors, IDs, etc., but the CSS, HTML, and JS are not tightly coupled enough for me to do this with any confidence.

I would love to get your thoughts on this process and if you have done anything similar and had better results.

I lead the Chrome Developer Relations team at Google.

We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.

Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.

I love to learn about what you are building, and how I can help with Chrome or Web development in general, so if you want to chat with me directly, please feel free to book a consultation.

I'm trialing a newsletter, you can subscribe below (thank you!)