Css :defined Example

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;
}

Syntax

:defined

Examples

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:

customElements.define('simple-custom',
  class extends HTMLElement {
    constructor() {
      super();

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

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

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

Specifications

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

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
:defined	54	79	63	No	41	10	54	54	63	41	10	6.0

See also

• Web components