1. 4.13 Custom elements
      1. 4.13.1 Introduction
        1. Creating an autonomous custom element
        2. Creating a customized built-in element
        3. Drawbacks of autonomous custom elements
        4. Upgrading elements after their creation
      2. 4.13.2 Requirements for custom element constructors and reactions
      3. 4.13.3 Core concepts
      4. 4.13.4 The CustomElementRegistry interface
      5. 4.13.5 Custom element reactions

4.13 Custom elements

Support: custom-elementsv1Chrome for Android 73+Chrome 67+iOS Safari (limited) 10.3+UC Browser for Android 11.8+Firefox 63+Samsung Internet 6.2+IE NoneSafari (limited) 10.1+Opera Mini NoneOpera (limited) 41+Android Browser 67+

Source: caniuse.com

4.13.1 Introduction

Custom elements provide a way for authors to build their own fully-featured DOM elements. Although authors could always use non-standard elements in their documents, with application-specific behavior added after the fact by scripting or similar, such elements have historically been non-conforming and not very functional. By defining a custom element, authors can inform the parser how to properly construct an element and how elements of that class should react to changes.

Custom elements are part of a larger effort to "rationalise the platform", by explaining existing platform features (like the elements of HTML) in terms of lower-level author-exposed extensibility points (like custom element definition). Although today there are many limitations on the capabilities of custom elements—both functionally and semantically—that prevent them from fully explaining the behaviors of HTML's existing elements, we hope to shrink this gap over time. Creating an autonomous custom element

For the purposes of illustrating how to create an autonomous custom element, let's define a custom element that encapsulates rendering a small icon for a country flag. Our goal is to be able to use it like so:

<flag-icon country="nl"></flag-icon>

To do this, we first declare a class for the custom element, extending HTMLElement:

class FlagIcon extends HTMLElement {
  constructor() {
    this._countryCode = null;

  static get observedAttributes() { return ["country"]; }

  attributeChangedCallback(name, oldValue, newValue) {
    // name will always be "country" due to observedAttributes
    this._countryCode = newValue;
  connectedCallback() {

  get country() {
    return this._countryCode;
  set country(v) {
    this.setAttribute("country", v);

  _updateRendering() {
    // Left as an exercise for the reader. But, you'll probably want to
    // check this.ownerDocument.defaultView to see if we've been
    // inserted into a document with a browsing context, and avoid
    // doing any work if not.

We then need to use this class to define the element:

customElements.define("flag-icon", FlagIcon);

At this point, our above code will work! The parser, whenever it sees the flag-icon tag, will construct a new instance of our FlagIcon class, and tell our code about its new country attribute, which we then use to set the element's internal state and update its rendering (when appropriate).

You can also create flag-icon elements using the DOM API:

const flagIcon = document.createElement("flag-icon")
flagIcon.country = "jp"

Finally, we can also use the custom element constructor itself. That is, the above code is equivalent to:

const flagIcon = new FlagIcon()
flagIcon.country = "jp"
document.body.appendChild(flagIcon) Creating a customized built-in element

Customized built-in elements are a distinct kind of custom element, which are defined slightly differently and used very differently compared to autonomous custom elements. They exist to allow reuse of behaviors from the existing elements of HTML, by extending those elements with new custom functionality. This is important since many of the existing behaviors of HTML elements can unfortunately not be duplicated by using purely autonomous custom elements. Instead, customized built-in elements allow the installation of custom construction behavior, lifecycle hooks, and prototype chain onto existing elements, essentially "mixing in" these capabilities on top of the already-existing element.

Customized built-in elements require a distinct syntax from autonomous custom elements because user agents and other software key off an element's local name in order to identify the element's semantics and behavior. That is, the concept of customized built-in elements building on top of existing behavior depends crucially on the extended elements retaining their original local name.

In this example, we'll be creating a customized built-in element named plastic-button, which behaves like a normal button but gets fancy animation effects added whenever you click on it. We start by defining a class, just like before, although this time we extend HTMLButtonElement instead of HTMLElement:

class PlasticButton extends HTMLButtonElement {
  constructor() {

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

When defining our custom element, we have to also specify the extends option:

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

In general, the name of the element being extended cannot be determined simply by looking at what element interface it extends, as many elements share the same interface (such as q and blockquote both sharing HTMLQuoteElement).

To construct our customized built-in element from parsed HTML source text, we use the is attribute on a button element:

<button is="plastic-button">Click Me!</button>

Trying to use a customized built-in element as an autonomous custom element will not work; that is, <plastic-button>Click me?</plastic-button> will simply create an HTMLElement with no special behavior.

If you need to create a customized built-in element programmatically, you can use the following form of createElement():

const plasticButton = document.createElement("button", { is: "plastic-button" });
plasticButton.textContent = "Click me!";

And as before, the constructor will also work:

const plasticButton2 = new PlasticButton();
console.log(plasticButton2.localName);  // will output "button"
console.assert(plasticButton2 instanceof PlasticButton);
console.assert(plasticButton2 instanceof HTMLButtonElement);

Note that when creating a customized built-in element programmatically, the is attribute will not be present in the DOM, since it was not explicitly set. However, it will be added to the output when serializing:

console.log(plasticButton.outerHTML); // will output '<button is="plastic-button"></button>'

Regardless of how it is created, all the of the ways in which button is special apply to such "plastic buttons" as well: their focus behavior, ability to participate in form submission, the disabled attribute, and so on.

Customized built-in elements are designed to allow extension of existing HTML elements that have useful user-agent supplied behavior or APIs. As such, they can only extend existing HTML elements defined in this specification, and cannot extend legacy elements such as bgsound, blink, isindex, keygen, multicol, nextid, or spacer that have been defined to use HTMLUnknownElement as their element interface.

One reason for this requirement is future-compatibility: if a customized built-in element was defined that extended a currently-unknown element, for example combobox, this would prevent this specification from defining a combobox element in the future, as consumers of the derived customized built-in element would have come to depend on their base element having no interesting user-agent-supplied behavior. Drawbacks of autonomous custom elements

As specified below, and alluded to above, simply defining and using an element called taco-button does not mean that such elements represent buttons. That is, tools such as Web browsers, search engines, or accessibility technology will not automatically treat the resulting element as a button just based on its defined name.

To convey the desired button semantics to a variety of users, while still using an autonomous custom element, a number of techniques would need to be employed:

With these points in mind, a full-featured taco-button that took on the responsibility of conveying button semantics (including the ability to be disabled) might look something like this:

class TacoButton extends HTMLElement {
  static get observedAttributes() { return ["disabled"]; }

  constructor() {

    this.addEventListener("keydown", e => {
      if (e.keyCode === 32 || e.keyCode === 13) {
        this.dispatchEvent(new MouseEvent("click", {
          bubbles: true,
          cancelable: true

    this.addEventListener("click", e => {
      if (this.disabled) {

    this._observer = new MutationObserver(() => {
      this.setAttribute("aria-label", this.textContent);

  connectedCallback() {
    this.setAttribute("role", "button");
    this.setAttribute("tabindex", "0");

    this._observer.observe(this, {
      childList: true,
      characterData: true,
      subtree: true

  disconnectedCallback() {

  get disabled() {
    return this.hasAttribute("disabled");

  set disabled(v) {
    if (v) {
      this.setAttribute("disabled", "");
    } else {

  attributeChangedCallback() {
    // only is called for the disabled attribute due to observedAttributes
    if (this.disabled) {
      this.setAttribute("aria-disabled", "true");
    } else {
      this.setAttribute("tabindex", "0");
      this.setAttribute("aria-disabled", "false");

Even with this rather-complicated element definition, the element is not a pleasure to use for consumers: it will be continually "sprouting" tabindex and aria-* attributes of its own volition. This is because as of now there is no way to specify default accessibility semantics or focus behavior for custom elements, forcing the use of these attributes to do so (even though they are usually reserved for allowing the consumer to override default behavior).

In contrast, a simple customized built-in element, as shown in the previous section, would automatically inherit the semantics and behavior of the button element, with no need to implement these behaviors manually. In general, for any elements with nontrivial behavior and semantics that build on top of existing elements of HTML, customized built-in elements will be easier to develop, maintain, and consume. Upgrading elements after their creation

Because element definition can occur at any time, a non-custom element could be created, and then later become a custom element after an appropriate definition is registered. We call this process "upgrading" the element, from a normal element into a custom element.

Upgrades enable scenarios where it may be preferable for custom element definitions to be registered after relevant elements have been initially created, such as by the parser. They allow progressive enhancement of the content in the custom element. For example, in the following HTML document the element definition for img-viewer is loaded asynchronously:

<!DOCTYPE html>
<html lang="en">
<title>Image viewer example</title>

<img-viewer filter="Kelvin">
  <img src="images/tree.jpg" alt="A beautiful tree towering over an empty savannah">

<script src="js/elements/img-viewer.js" async></script>

The definition for the img-viewer element here is loaded using a script element marked with the async attribute, placed after the <img-viewer> tag in the markup. While the script is loading, the img-viewer element will be treated as an undefined element, similar to a span. Once the script loads, it will define the img-viewer element, and the existing img-viewer element on the page will be upgraded, applying the custom element's definition (which presumably includes applying an image filter identified by the string "Kelvin", enhancing the image's visual appearance).

Note that upgrades only apply to elements in the document tree. (Formally, elements that are connected.) An element that is not inserted into a document will stay un-upgraded. An example illustrates this point:

<!DOCTYPE html>
<html lang="en">
<title>Upgrade edge-cases example</title>


  "use strict";

  const inDocument = document.querySelector("example-element");
  const outOfDocument = document.createElement("example-element");

  // Before the element definition, both are HTMLElement:
  console.assert(inDocument instanceof HTMLElement);
  console.assert(outOfDocument instanceof HTMLElement);

  class ExampleElement extends HTMLElement {}
  customElements.define("example-element", ExampleElement);

  // After element definition, the in-document element was upgraded:
  console.assert(inDocument instanceof ExampleElement);
  console.assert(!(outOfDocument instanceof ExampleElement));


  // Now that we've moved the element into the document, it too was upgraded:
  console.assert(outOfDocument instanceof ExampleElement);

4.13.2 Requirements for custom element constructors and reactions

When authoring custom element constructors, authors are bound by the following conformance requirements:

Several of these requirements are checked during element creation, either directly or indirectly, and failing to follow them will result in a custom element that cannot be instantiated by the parser or DOM APIs. This is true even if the work is done inside a constructor-initiated microtask, as a microtask checkpoint can occur immediately after construction.

When authoring custom element reactions, authors should avoid manipulating the node tree as this can lead to unexpected results.

An element's connectedCallback can be queued before the element is disconnected, but as the callback queue is still processed, it results in a connectedCallback for an element that is no longer connected:

class CParent extends HTMLElement {
  connectedCallback() {
customElements.define("c-parent", CParent);

class CChild extends HTMLElement {
  connectedCallback() {
    console.log("CChild connectedCallback: isConnected =", this.isConnected);
customElements.define("c-child", CChild);

const parent = new CParent(),
      child = new CChild();

// Logs:
// CChild connectedCallback: isConnected = false

4.13.3 Core concepts

A custom element is an element that is custom. Informally, this means that its constructor and prototype are defined by the author, instead of by the user agent. This author-supplied constructor function is called the custom element constructor.

Two distinct types of custom elements can be defined:

  1. An autonomous custom element, which is defined with no extends option. These types of custom elements have a local name equal to their defined name.

  2. A customized built-in element, which is defined with an extends option. These types of custom elements have a local name equal to the value passed in their extends option, and their defined name is used as the value of the is attribute, which therefore must be a valid custom element name.

After a custom element is created, changing the value of the is attribute does not change the element's behavior.

Autonomous custom elements have the following element definition:

Flow content.
Phrasing content.
Palpable content.
Contexts in which this element can be used:
Where phrasing content is expected.
Content model:
Content attributes:
Global attributes, except the is attribute
Any other attribute that has no namespace (see prose).
DOM interface:
Supplied by the element's author (inherits from HTMLElement)

An autonomous custom element does not have any special meaning: it represents its children. A customized built-in element inherits the semantics of the element that it extends.

Any namespace-less attribute that is relevant to the element's functioning, as determined by the element's author, may be specified on an autonomous custom element, so long as the attribute name is XML-compatible and contains no ASCII upper alphas. The exception is the is attribute, which must not be specified on an autonomous custom element (and which will have no effect if it is).

Customized built-in elements follow the normal requirements for attributes, based on the elements they extend. To add custom attribute-based behavior, use data-* attributes.

A valid custom element name is a sequence of characters name that meets all of the following requirements:

These requirements ensure a number of goals for valid custom element names:

Apart from these restrictions, a large variety of names is allowed, to give maximum flexibility for use cases like <math-α> or <emotion-😍>.

4.13.4 The CustomElementRegistry interface

Custom element registries are associated with Window objects, instead of Document objects, since each custom element constructor inherits from the HTMLElement interface, and there is exactly one HTMLElement interface per Window object.

window . customElements . define(name, constructor)
Defines a new custom element, mapping the given name to the given constructor as an autonomous custom element.
window . customElements . define(name, constructor, { extends: baseLocalName })
Defines a new custom element, mapping the given name to the given constructor as a customized built-in element for the element type identified by the supplied baseLocalName. A "NotSupportedError" DOMException will be thrown upon trying to extend a custom element or an unknown element.
window . customElements . get(name)
Retrieves the custom element constructor defined for the given name. Returns undefined if there is no custom element definition with the given name.
window . customElements . whenDefined(name)
Returns a promise that will be fulfilled when a custom element becomes defined with the given name. (If such a custom element is already defined, the returned promise will be immediately fulfilled.) Returns a promise rejected with a "SyntaxError" DOMException if not given a valid custom element name.
window . customElements . upgrade(root)
Tries to upgrade all shadow-including inclusive descendant elements of root, even if they are not connected.

Element definition is a process of adding a custom element definition to the CustomElementRegistry. This is accomplished by the define() method.

The whenDefined() method can be used to avoid performing an action until all appropriate custom elements are defined. In this example, we combine it with the :defined pseudo-class to hide a dynamically-loaded article's contents until we're sure that all of the autonomous custom elements it uses are defined.

articleContainer.hidden = true;

  .then(response => response.text())
  .then(text => {
    articleContainer.innerHTML = text;

    return Promise.all(
        .map(el => customElements.whenDefined(el.localName))
  .then(() => {
    articleContainer.hidden = false;

The upgrade() method allows upgrading of elements at will. Normally elements are automatically upgraded when they become connected, but this method can be used if you need to upgrade before you're ready to connect the element.

const el = document.createElement("spider-man");

class SpiderMan extends HTMLElement {}
customElements.define("spider-man", SpiderMan);

console.assert(!(el instanceof SpiderMan)); // not yet upgraded

console.assert(el instanceof SpiderMan);    // upgraded!

4.13.5 Custom element reactions

A custom element possesses the ability to respond to certain occurrences by running author code:

We call these reactions collectively custom element reactions.

The way in which custom element reactions are invoked is done with special care, to avoid running author code during the middle of delicate operations. Effectively, they are delayed until "just before returning to user script". This means that for most purposes they appear to execute synchronously, but in the case of complicated composite operations (like cloning, or range manipulation), they will instead be delayed until after all the relevant user agent processing steps have completed, and then run together as a batch.

It is guaranteed that custom element reactions always are invoked in the same order as their triggering actions, at least within the local context of a single custom element. (Because custom element reaction code can perform its own mutations, it is not possible to give a global ordering guarantee across multiple elements.)