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;




This example first includes a script defining a <simple-custom> custom element:

  class extends HTMLElement {
    constructor() {

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

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

Next, we have some HTML with an instance of the <simple-custom> element, and a standard <p> element:

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

<p>Standard paragraph example text</p>

Now for some CSS. Here we define background colors based on element types, change the opacity of the custom element based on whether or not it is defined, and use the :defined selector to make all defined element text appear in italic.

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

simple-custom {
  display: block;
  background: cyan;

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

Finally, we 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) {
  opacity: 0;

simple-custom:defined {
  opacity: 0.75;
  text-decoration: underline;

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.


Here is the result of running the code above:


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
Basic support Yes ? 63 No Yes Yes
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support Yes Yes ? 63 Yes Yes Yes

See also

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