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Detecting when a Web Component is loaded with CSS

Over the last week or so, I’ve been writing about why Web Components are awesome.

Today, I wanted to share an awesome little trick Hawk Ticehurst taught me over on Mastodon.

The CSS :defined pseudo-class applies to an element when it’s been defined in the browser. For Web Components, that doesn’t happen until the customElements.define() method has been run on your custom element.

<!-- This is a counter button -->
// The <count-up> element receives the :defined pseudo-class when this JS method runs
customElements.define('count-up', class extends HTMLElement {
	// ...

I’m a big fan of HTML Web Components, custom elements that progressively enhanced existing functional HTML.

But sometimes, a Web Component will only work with JavaScript and won’t work until its been defined.

👋 Quick aside: if you want to learn how to create your own Web Components, I have 17 lessons, a workshop, a handful of projects, and tons of demos over at the Lean Web Club. Today only, you can join for less than $5/month, or just $45 for a whole year!

Historically, I’ve added a [hidden] attribute to the custom element, and removed it as part of the instantiation process in the constructor() method for my Web Component class.

<!-- Hide it by default -->
<count-up hidden></count-up>
// Show it when the element is instantiated
customElements.define('count-up', class extends HTMLElement {
	constructor () {

But the :defined pseudo-class provides another way to do this using just CSS instead of JavaScript.

/* Hide the <count-up> element until it's defined */
count-up:not(:defined) {
	display: none;

Here’s a demo.