What is HTML "is" attribute?
It is part of the W3C Draft spec for Web Components' Custom Elements.
Latest Working Draft: http://www.w3.org/TR/custom-elements/#type-extension-semantics
Latest Editor's Draft: http://w3c.github.io/webcomponents/spec/custom/#type-extension-example
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 <input>, <button> or <table>. For example, you could have a plastic-button element that extends <button> to provide some fancy animation when clicked.
You'd add the button to the page like this:
<button is="plastic-button">Click Me!</button>
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 <plastic-button>Click Me!</plastic-button> instead of <button is="plastic-button">Click Me!</button>, but that would create an HTMLElement with no special behavior.
If you are NOT extending a built-in HTML element like <button> and instead creating a new element that extends the generic HTMLElement, you can use the <plastic-button> syntax. But you won't get any of <button>'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
In 2020:
The is attribute is now part of HTML spec in the Custom Elements specification.
It follows the polymer spec and is documented for developers at mdn.
Only Edge still hasn't updated to include this spec but it its new chromium-based implementation, in 2020, its availability may become widespread.
In 2017:
There is no is attribute in HTML.
It is a proposed extension that appears in the Custom Elements specification (which evolved from the Polymer spec mentioned below).
It allows you to say that an existing, standard element is really a custom element.
<button is="fancy-button" disabled>Fancy button!</button>
… which allows for backwards compatibility. If custom elements are not supported by the browser (the spec is still a draft and has very limited browser support) then it will fall back to the default behaviour.
In 2014:
It is not HTML. It is an expando-attribute for Polymer custom elements.
If you used extends to create a Polymer element that derives from an existing DOM element (something other than HTMLElement), use the
issyntax