CSS - Pseudo-class :defined

The CSS pseudo-class selector :defined represents an element that has been defined, using the customElementRegistry.define() method. This includes standard elements, as well as the custom elements.

Syntax

:defined {
   /* ... */
}

CSS :defined Example

Following example demonstrates use of :defined pseudo-class:

<html>
<head>
<style>
   /* standard element p given a background color, whereas custom element has no background-color set */
   p {
      background-color: yellow;
      font-family: 'Courier New', Courier, monospace;
   }

   sample-custom-element {
      color: blue;
   }

   /* Both the standard and custom element is given the font-style as normal, font-weight as 700 and font-family as fantasy */
   :defined {
      font-style: normal;
      font-weight: 700;
      font-family: fantasy; 
   }
</style>
</head>
<body>
   <h3><u>:defined example</u></h3>  

   <sample-custom-element text="Custom element with font color blue and other styles as per the :defined pseudo-class"></sample-custom-element>

   <p>Standard p element with yellow background, and font as per the :defined pseudo-class</p>

   <script>
      customElements.define(
         "sample-custom-element",
         class extends HTMLElement {
         constructor() {
         super();

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

         let shadowRoot = this.attachShadow({ mode: "open" }).appendChild(divElem);
         }
         },
      );
   </script>
</body>
</html>

In the above example:

A standard element <p> is given some styling (background-color: yellow; font-family: 'Courier New', Courier, monospace;)
A custom element (sample-custom-element) has been declared and is given some styling (color: blue;)
Pseudo-class :defined is applied on the defined elements, which has styling specified as (font-style: normal; font-weight: 700; font-family: fantasy;)
The output shows that both the elements, standard and custom, takes up the styling specified using the :defined pseudo-class.