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;




Hiding elements until they are defined

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


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
:defined 54 79 63 No 41 10
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
:defined 54 54 63 41 10 6.0

See also

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