What is HTML “is” attribute?

Question

I have seen it a lot here and there, yet I could find any description or documentation about it!

Example:

Answer 1

The is keyword is part of the W3C Draft spec for creating custom HTML elements with custom behavior.

In specific, is is used when extending a built-in element like , or . For example, you could have a plastic-button element that extends to provide some fancy animation when clicked.

You'd add the button to the page like this:

Click Me!

Before you do this, you need to register plastic-button as a custom HTML element like this:

customElements.define("plastic-button", PlasticButton, { extends: "button" });

This references a PlasticButton Javascript class, which would look something like this:

class PlasticButton extends HTMLButtonElement {
  constructor() {
    super();

    this.addEventListener("click", () => {
      // Draw some fancy animation effects!
    });
  }
}

It'd be great if you could say Click Me! instead of Click Me!, but that would create an HTMLElement with no special behavior.

If you are NOT extending a built-in HTML element like and instead creating a new element that extends the generic HTMLElement, you can use the syntax. But you won't get any of 's behavior.

This is part of the W3C Draft spec for Web Components' Custom Elements: http://www.w3.org/TR/custom-elements/#type-extension-semantics

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