The :defined CSS pseudo-class represents any element that has been defined. This includes any standard element built in to the browser, and custom elements that have been successfully defined (i.e. with the CustomElementRegistry.define() method).

/* Selects any defined element */
:defined {
  font-style: italic;

/* Selects any instance of a specific custom element */
simple-custom:defined {
  display: block;




The following snippets are taken from our defined-pseudo-class demo (see it live also).

In this demo we define a very simple trivial custom element:

  class extends HTMLElement {
    constructor() {

      let divElem = document.createElement('div');
      divElem.textContent = this.getAttribute('text');

      let shadowRoot = this.attachShadow({mode: 'open'})

Then insert a copy of this element into the document, along with a standard <p>:

<simple-custom text="Custom element example text"></simple-custom>

<p>Standard paragraph example text</p>

In the CSS we first include the following rules:

// Give the two elements distinctive backgrounds
p {
  background: yellow;

simple-custom {
  background: cyan;

// Both the custom and the built-in element are given italic text
:defined {
  font-style: italic;

Then provide the following two rules to hide any instances of our custom element that are not defined, and display instances that are defined as block level elements:

simple-custom:not(:defined) {
  display: none;

simple-custom:defined {
  display: block;

This is useful if you have a complex custom element that takes a while to load into the page — you might want to hide instances of the element until definition is complete, so that you don't end up with flashes of ugly unstyled elements on the page.


Specification Status Comment
HTML Living Standard
The definition of ':defined' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes ? No No Yes Yes
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes ? No Yes Yes Yes

See also

Document Tags and Contributors

Contributors to this page: jpmedley, xfq, fscholz, chrisdavidmills
Last updated by: jpmedley,