1. 4.13 Custom elements
      1. 4.13.1 Introduction
        1. 4.13.1.1 Creating an autonomous custom element
        2. 4.13.1.2 Creating a form-associated custom element
        3. 4.13.1.3 Creating a custom element with default accessible roles, states, and properties
        4. 4.13.1.4 Creating a customized built-in element
        5. 4.13.1.5 Drawbacks of autonomous custom elements
        6. 4.13.1.6 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 Upgrades
      6. 4.13.6 Custom element reactions
      7. 4.13.7 Element internals
        1. 4.13.7.1 The ElementInternals interface
        2. 4.13.7.2 Shadow root access
        3. 4.13.7.3 Form-associated custom elements
        4. 4.13.7.4 Accessibility semantics

4.13 Custom elements

Using_custom_elements

Support in all current engines.

Firefox 63+ Safari 10.1+ Chrome 54+
Opera 41+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 10.3+ Chrome Android 54+ WebView Android 54+ Samsung Internet 6.0+ Opera Android 41+
caniuse.com table

4.13.1 Introduction

This section is non-normative.

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.

4.13.1.1 Creating an autonomous custom element

This section is non-normative.

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() {
    super();
    this._countryCode = null;
  }
  static get observedAttributes() { return ["country"]; }
  attributeChangedCallback(name, oldValue, newValue) {
    // name will always be "country" due to observedAttributes
    this._countryCode = newValue;
    this._updateRendering();
  }
  connectedCallback() {
    this._updateRendering();
  }
  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"
document.body.appendChild(flagIcon)

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)
4.13.1.2 Creating a form-associated custom element

This section is non-normative.

Adding a static formAssociated property, with a true value, makes an autonomous custom element a form-associated custom element . The ElementInternals interface helps you to implement functions and properties common to form control elements. 

class MyCheckbox extends HTMLElement {
  static get formAssociated() { return true; }
  static get observedAttributes() { return ['checked']; }
  constructor() {
    super();
    this._internals = this.attachInternals();
    this.addEventListener('click', this._onClick.bind(this));
  }
  get form() { return this._internals.form; }
  get name() { return this.getAttribute('name'); }
  get type() { return this.localName; }
  get checked() { return this.getAttribute('checked'); }
  set checked(flag) { this.toggleAttribute('checked', Boolean(flag)); }
  attributeChangedCallback(name, oldValue, newValue) {
    // name will always be "checked" due to observedAttributes
    this._internals.setFormValue(this.checked ? 'on' : null);
  }
  _onClick(event) {
    this.checked = !this.checked;
  }
}
customElements.define('my-checkbox',
MyCheckbox);

You can use the custom element my-checkbox like a built-in form-associated element. For example, putting it in form or label associates the my-checkbox element with them, and submitting the form will send data provided by my-checkbox implementation. 

<form action="..." method="...">
  <label><my-checkbox name="agreed"></my-checkbox> I read the agreement.</label>
  <input type="submit">
</form>
4.13.1.3 Creating a custom element with default accessible roles, states, and properties

This section is non-normative.

By using the appropriate properties of ElementInternals , your custom element can have default accessibility semantics. The following code expands our form-associated checkbox from the previous section to properly set its default role and checkedness, as viewed by accessibility technology: 

class MyCheckbox extends HTMLElement {
  static get formAssociated() { return true; }
  static get observedAttributes() { return ['checked']; }
  constructor() {
    super();
    this._internals = this.attachInternals();
    this.addEventListener('click', this._onClick.bind(this));
    this._internals.role = 'checkbox';
    this._internals.ariaChecked = false;
  }
  get form() { return this._internals.form; }
  get name() { return this.getAttribute('name'); }
  get type() { return this.localName; }
  get checked() { return this.getAttribute('checked'); }
  set checked(flag) { this.toggleAttribute('checked', Boolean(flag)); }
  attributeChangedCallback(name, oldValue, newValue) {
    // name will always be "checked" due to observedAttributes
    this._internals.setFormValue(this.checked ? 'on' : null);
    this._internals.ariaChecked = this.checked;
  }
  _onClick(event) {
    this.checked = !this.checked;
  }
}
customElements.define('my-checkbox',
MyCheckbox);

Note that, like for built-in elements, these are only defaults, and can be overridden by the page author using the role and aria-* attributes: 

<!-- This markup is non-conforming -->
<input
type="checkbox"
checked
role="button"
aria-checked="false">


<!-- This markup is probably not what the custom element author intended -->
<my-checkbox
role="button"
checked
aria-checked="false">

Custom element authors are encouraged to state what aspects of their accessibility semantics are strong native semantics, i.e., should not be overridden by users of the custom element. In our example, the author of the my-checkbox element would state that its role and aria-checked values are strong native semantics, thus discouraging code such as the above.

4.13.1.4 Creating a customized built-in element

This section is non-normative.

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

Regardless of how it is created, all 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.

4.13.1.5 Drawbacks of autonomous custom elements

This section is non-normative.

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() {
    super();
    this._internals = this.attachInternals();
    this._internals.role = "button";
    this.addEventListener("keydown", e => {
      if (e.code === "Enter" || e.code === "Space") {
        this.dispatchEvent(new PointerEvent("click", {
          bubbles: true,
          cancelable: true
        }));
      }
    });
    this.addEventListener("click", e => {
      if (this.disabled) {
        e.preventDefault();
        e.stopImmediatePropagation();
      }
    });
    this._observer = new MutationObserver(() => {
      this._internals.ariaLabel = this.textContent;
    });
  }
  connectedCallback() {
    this.setAttribute("tabindex", "0");
    this._observer.observe(this, {
      childList: true,
      characterData: true,
      subtree: true
    });
  }
  disconnectedCallback() {
    this._observer.disconnect();
  }
  get disabled() {
    return this.hasAttribute("disabled");
  }
  set disabled(flag) {
    this.toggleAttribute("disabled", Boolean(flag));
  }
  attributeChangedCallback(name, oldValue, newValue) {
    // name will always be "disabled" due to observedAttributes
    if (this.disabled) {
      this.removeAttribute("tabindex");
      this._internals.ariaDisabled = "true";
    } else {
      this.setAttribute("tabindex", "0");
      this._internals.ariaDisabled = "false";
    }
  }
}

Even with this rather-complicated element definition, the element is not a pleasure to use for consumers: it will be continually "sprouting" tabindex attributes of its own volition, and its choice of tabindex="0" focusability behavior may not match the button behavior on the current platform. This is because as of now there is no way to specify default focus behavior for custom elements, forcing the use of the tabindex attribute to do so (even though it is 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.

4.13.1.6 Upgrading elements after their creation

This section is non-normative.

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">
</img-viewer>
<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>
<example-element></example-element>
<script>
  "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));
  document.body.appendChild(outOfDocument);
  // Now that we've moved the element into the document, it too was upgraded:
  console.assert(outOfDocument instanceof ExampleElement);
</script>

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() {
    this.firstChild.remove();
  }
}
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();
parent.append(child);
document.body.append(parent);
// 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:

Global_attributes/is

Firefox 63+ Safari No Chrome 67+
Opera 55+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS No Chrome Android 67+ WebView Android 67+ Samsung Internet 9.0+ Opera Android 48+

  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, as it is saved on the element as its is value .

Autonomous custom elements have the following element definition:

Categories :
Flow content .
Phrasing content .
Palpable content .
For form-associated custom elements : Listed , labelable , submittable , and resettable form-associated element .
Contexts in which this element can be used :
Where phrasing content is expected.
Content model :
Transparent .
Content attributes :
Global attributes , except the is attribute
form , for form-associated custom elements
disabled , for form-associated custom elements
readonly , for form-associated custom elements
name , for form-associated custom elements
Any other attribute that has no namespace (see prose).
Accessibility considerations :
For form-associated custom elements : for authors ; for implementers .
Otherwise: for authors ; for implementers .
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.

An autonomous custom element is called a form-associated custom element if the element is associated with a custom element definition whose form-associated field is set to true.

The name attribute represents the form-associated custom element 's name. The disabled attribute is used to make the form-associated custom element non-interactive and to prevent its submission value from being submitted. The form attribute is used to explicitly associate the form-associated custom element with its form owner .

The readonly attribute of form-associated custom elements specifies that the element is barred from constraint validation . User agents don't provide any other behavior for the attribute, but custom element authors should, where possible, use its presence to make their control non-editable in some appropriate fashion, similar to the behavior for the readonly attribute on built-in form controls.

Constraint validation : If the readonly attribute is specified on a form-associated custom element , the element is barred from constraint validation .

The reset algorithm for form-associated custom elements is to enqueue a custom element callback reaction with the element, callback name " formResetCallback ", and an empty argument list.

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-😍> .

A custom element definition describes a custom element and consists of:

A name
A valid custom element name
A local name
A local name
A constructor
A Web IDL CustomElementConstructor callback function type value wrapping the custom element constructor
A list of observed attributes
A sequence<DOMString>
A collection of lifecycle callbacks
A map, whose keys are the strings " connectedCallback ", " disconnectedCallback ", " adoptedCallback ", " attributeChangedCallback ", " formAssociatedCallback ", " formDisabledCallback ", " formResetCallback ", and " formStateRestoreCallback ". The corresponding values are either a Web IDL Function callback function type value, or null. By default the value of each entry is null.
A construction stack
A list, initially empty, that is manipulated by the upgrade an element algorithm and the HTML element constructors . Each entry in the list will be either an element or an already constructed marker .
A form-associated boolean
If this is true, user agent treats elements associated to this custom element definition as form-associated custom elements .
A disable internals boolean
Controls attachInternals() .
A disable shadow boolean
Controls attachShadow() .

To look up a custom element definition , given a document , namespace , localName , and is , perform the following steps. They will return either a custom element definition or null:

  1. If namespace is not the HTML namespace , return null.

  2. If document 's browsing context is null, return null.

  3. Let registry be document 's relevant global object 's CustomElementRegistry object.

  4. If there is custom element definition in registry with name and local name both equal to localName , return that custom element definition .

  5. If there is a custom element definition in registry with name equal to is and local name equal to localName , return that custom element definition .

  6. Return null.

4.13.4 The CustomElementRegistry interface

Each Window object is associated with a unique instance of a CustomElementRegistry object, allocated when the Window object is created.

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

Support in all current engines.

Firefox 63+ Safari 10.1+ Chrome 54+
Opera 41+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 10.3+ Chrome Android 54+ WebView Android 54+ Samsung Internet 6.0+ Opera Android 41+

The customElements attribute of the Window interface must return the CustomElementRegistry object for that Window object.

CustomElementRegistry

Support in all current engines.

Firefox63+Safari10.1+Chrome54+
Opera41+Edge79+
Edge (Legacy)NoInternet ExplorerNo
Firefox Android63+Safari iOS10.3+Chrome Android54+WebView Android54+Samsung Internet6.0+Opera Android41+
[Exposed=Window]
interface CustomElementRegistry {
  [CEReactions] undefined define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions options = {});
  (CustomElementConstructor or undefined) get(DOMString name);
  Promise<CustomElementConstructor> whenDefined(DOMString name);
  [CEReactions] undefined upgrade(Node root);
};
callback CustomElementConstructor = HTMLElement ();
dictionary ElementDefinitionOptions {
  DOMString extends;
};

Every CustomElementRegistry has a set of custom element definitions , initially empty. In general, algorithms in this specification look up elements in the registry by any of name , local name , or constructor .

Every CustomElementRegistry also has an element definition is running flag which is used to prevent reentrant invocations of element definition . It is initially unset.

Every CustomElementRegistry also has a when-defined promise map , mapping valid custom element names to promises. It is used to implement the whenDefined() method.

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 with the custom element 's constructor 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 .

CustomElementRegistry/define

Support in all current engines.

Firefox 63+ Safari 10.1+ Chrome 66+
Opera 53+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 10.3+ Chrome Android 66+ WebView Android 66+ Samsung Internet 9.0+ Opera Android 47+

Element definition is a process of adding a custom element definition to the CustomElementRegistry . This is accomplished by the define() method. When invoked, the define( name , constructor , options ) method must run these steps:

  1. If IsConstructor ( constructor ) is false, then throw a TypeError .

  2. If name is not a valid custom element name , then throw a " SyntaxError " DOMException .

  3. If this CustomElementRegistry contains an entry with name name , then throw a " NotSupportedError " DOMException .

  4. If this CustomElementRegistry contains an entry with constructor constructor , then throw a " NotSupportedError " DOMException .

  5. Let localName be name .

  6. Let extends be the value of the extends member of options , or null if no such member exists.

  7. If extends is not null, then:

    1. If extends is a valid custom element name , then throw a " NotSupportedError " DOMException .

    2. If the element interface for extends and the HTML namespace is HTMLUnknownElement (e.g., if extends does not indicate an element definition in this specification), then throw a " NotSupportedError " DOMException .

    3. Set localName to extends .

  8. If this CustomElementRegistry 's element definition is running flag is set, then throw a " NotSupportedError " DOMException .

  9. Set this CustomElementRegistry 's element definition is running flag.

  10. Let formAssociated be false.

  11. Let disableInternals be false.

  12. Let disableShadow be false.

  13. Let observedAttributes be an empty sequence<DOMString> .

  14. Run the following substeps while catching any exceptions:

    1. Let prototype be Get ( constructor , "prototype"). Rethrow any exceptions.

    2. If Type ( prototype ) is not Object, then throw a TypeError exception.

    3. Let lifecycleCallbacks be a map with the keys " connectedCallback ", " disconnectedCallback ", " adoptedCallback ", and " attributeChangedCallback ", each of which belongs to an entry whose value is null.

    4. For each of the keys callbackName in lifecycleCallbacks , in the order listed in the previous step:

      1. Let callbackValue be Get ( prototype , callbackName ). Rethrow any exceptions.

      2. If callbackValue is not undefined, then set the value of the entry in lifecycleCallbacks with key callbackName to the result of converting callbackValue to the Web IDL Function callback type. Rethrow any exceptions from the conversion.

    5. If the value of the entry in lifecycleCallbacks with key " attributeChangedCallback " is not null, then:

      1. Let observedAttributesIterable be Get ( constructor , "observedAttributes"). Rethrow any exceptions.

      2. If observedAttributesIterable is not undefined, then set observedAttributes to the result of converting observedAttributesIterable to a sequence<DOMString> . Rethrow any exceptions from the conversion.

    6. Let disabledFeatures be an empty sequence<DOMString> .

    7. Let disabledFeaturesIterable be Get ( constructor , "disabledFeatures"). Rethrow any exceptions.

    8. If disabledFeaturesIterable is not undefined, then set disabledFeatures to the result of converting disabledFeaturesIterable to a sequence<DOMString> . Rethrow any exceptions from the conversion.

    9. Set disableInternals to true if disabledFeatures contains " internals ".

    10. Set disableShadow to true if disabledFeatures contains " shadow ".

    11. Let formAssociatedValue be Get ( constructor , "formAssociated"). Rethrow any exceptions.

    12. Set formAssociated to the result of converting formAssociatedValue to a boolean . Rethrow any exceptions from the conversion.

    13. If formAssociated is true, for each of " formAssociatedCallback ", " formResetCallback ", " formDisabledCallback ", and " formStateRestoreCallback " callbackName :

      1. Let callbackValue be Get ( prototype , callbackName ). Rethrow any exceptions.

      2. If callbackValue is not undefined, then set the value of the entry in lifecycleCallbacks with key callbackName to the result of converting callbackValue to the Web IDL Function callback type. Rethrow any exceptions from the conversion.

    Then, perform the following substep, regardless of whether the above steps threw an exception or not:

    1. Unset this CustomElementRegistry 's element definition is running flag.

    Finally, if the first set of substeps threw an exception, then rethrow that exception (thus terminating this algorithm). Otherwise, continue onward.

  15. Let definition be a new custom element definition with name name , local name localName , constructor constructor , observed attributes observedAttributes , lifecycle callbacks lifecycleCallbacks , form-associated formAssociated , disable internals disableInternals , and disable shadow disableShadow .

  16. Add definition to this CustomElementRegistry .

  17. Let document be this CustomElementRegistry 's relevant global object 's associated Document .

  18. Let upgrade candidates be all elements that are shadow-including descendants of document , whose namespace is the HTML namespace and whose local name is localName , in shadow-including tree order . Additionally, if extends is non-null, only include elements whose is value is equal to name .

  19. For each element element in upgrade candidates , enqueue a custom element upgrade reaction given element and definition .

  20. If this CustomElementRegistry 's when-defined promise map contains an entry with key name :

    1. Let promise be the value of that entry.

    2. Resolve promise with constructor .

    3. Delete the entry with key name from this CustomElementRegistry 's when-defined promise map .

CustomElementRegistry/get

Support in all current engines.

Firefox 63+ Safari 10.1+ Chrome 66+
Opera 53+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 10.3+ Chrome Android 66+ WebView Android 66+ Samsung Internet 9.0+ Opera Android 47+

When invoked, the get( name ) method must run these steps:

  1. If this CustomElementRegistry contains an entry with name name , then return that entry's constructor .

  2. Otherwise, return undefined.

CustomElementRegistry/whenDefined

Support in all current engines.

Firefox 63+ Safari 10.1+ Chrome 66+
Opera 53+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 10.3+ Chrome Android 66+ WebView Android 66+ Samsung Internet 9.0+ Opera Android 47+

When invoked, the whenDefined( name ) method must run these steps:

  1. If name is not a valid custom element name , then return a new promise rejected with a " SyntaxError " DOMException .

  2. If this CustomElementRegistry contains an entry with name name , then return a new promise resolved with that entry's constructor .

  3. Let map be this CustomElementRegistry 's when-defined promise map .

  4. If map does not contain an entry with key name , create an entry in map with key name and whose value is a new promise.

  5. Let promise be the value of the entry in map with key name .

  6. Return promise .

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;
fetch(articleURL)
  .then(response => response.text())
  .then(text => {
    articleContainer.innerHTML = text;
    return Promise.all(
      [...articleContainer.querySelectorAll(":not(:defined)")]
        .map(el => customElements.whenDefined(el.localName))
    );
  })
  .then(() => {
    articleContainer.hidden = false;
});

CustomElementRegistry/upgrade

Support in all current engines.

Firefox 63+ Safari 12.1+ Chrome 68+
Opera 55+ Edge 79+
Edge (Legacy) No Internet Explorer No
Firefox Android 63+ Safari iOS 12.2+ Chrome Android 68+ WebView Android 68+ Samsung Internet 10.0+ Opera Android 48+

When invoked, the upgrade( root ) method must run these steps:

  1. Let candidates be a list of all of root 's shadow-including inclusive descendant elements, in shadow-including tree order .

  2. For each candidate of candidates , try to upgrade candidate .

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

4.13.5 Upgrades

To upgrade an element , given as input a custom element definition definition and an element element , run the following steps:

  1. If element 's custom element state is not " undefined " or " uncustomized ", then return.

    One scenario where this can occur due to reentrant invocation of this algorithm, as in the following example: 

    <!DOCTYPE html>
<x-foo id="a"></x-foo>
<x-foo id="b"></x-foo>
<script>
// Defining enqueues upgrade reactions for both "a" and "b"
customElements.define("x-foo", class extends HTMLElement {
  constructor() {
    super();
    const b = document.querySelector("#b");
    b.remove();
    // While this constructor is running for "a", "b" is still
    // undefined, and so inserting it into the document will enqueue a
    // second upgrade reaction for "b" in addition to the one enqueued
    // by defining x-foo.
    document.body.appendChild(b);
  }
})
</script>

    This step will thus bail out the algorithm early when upgrade an element is invoked with " b " a second time.

  2. Set element 's custom element definition to definition .

  3. Set element 's custom element state to " failed ".

    It will be set to " custom " after the upgrade succeeds . For now, we set it to " failed " so that any reentrant invocations will hit the above early-exit step .

  4. For each attribute in element 's attribute list , in order, enqueue a custom element callback reaction with element , callback name " attributeChangedCallback ", and an argument list containing attribute 's local name, null, attribute 's value, and attribute 's namespace.

  5. If element is connected , then enqueue a custom element callback reaction with element , callback name " connectedCallback ", and an empty argument list.

  6. Add element to the end of definition 's construction stack .

  7. Let C be definition 's constructor .

  8. Run the following substeps while catching any exceptions:

    1. If definition 's disable shadow is true and element 's shadow root is non-null, then throw a " NotSupportedError " DOMException .

      This is needed as attachShadow() does not use look up a custom element definition while attachInternals() does.

    2. Set element 's custom element state to " precustomized ".

    3. Let constructResult be the result of constructing C , with no arguments.

      If C non-conformantly uses an API decorated with the [CEReactions] extended attribute, then the reactions enqueued at the beginning of this algorithm will execute during this step, before C finishes and control returns to this algorithm. Otherwise, they will execute after C and the rest of the upgrade process finishes.

    4. If SameValue ( constructResult , element ) is false, then throw a TypeError .

      This can occur if C constructs another instance of the same custom element before calling super() , or if C uses JavaScript's return -override feature to return an arbitrary HTMLElement object from the constructor.

    Then, perform the following substep, regardless of whether the above steps threw an exception or not:

    1. Remove the last entry from the end of definition 's construction stack .

      Assuming C calls super() (as it will if it is conformant ), and that the call succeeds, this will be the already constructed marker that replaced the element we pushed at the beginning of this algorithm. (The HTML element constructor carries out this replacement.)

      If C does not call super() (i.e. it is not conformant ), or if any step in the HTML element constructor throws, then this entry will still be element .

    Finally, if the above steps threw an exception, then:

    1. Set element 's custom element definition to null.

    2. Empty element 's custom element reaction queue .

    3. Rethrow the exception (thus terminating this algorithm).

    If the above steps threw an exception, then element 's custom element state will remain " failed " or " precustomized ".

  9. If element is a form-associated custom element , then:

    1. Reset the form owner of element . If element is associated with a form element, then enqueue a custom element callback reaction with element , callback name " formAssociatedCallback ", and « the associated form ».

    2. If element is disabled , then enqueue a custom element callback reaction with element , callback name " formDisabledCallback " and « true ».

  10. Set element 's custom element state to " custom ".

To try to upgrade an element , given as input an element element , run the following steps:

  1. Let definition be the result of looking up a custom element definition given element 's node document , element 's namespace, element 's local name, and element 's is value .

  2. If definition is not null, then enqueue a custom element upgrade reaction given element and definition .

4.13.6 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.

Additionally, the precise ordering of these reactions is managed via a somewhat-complicated stack-of-queues system, described below. The intention behind this system is to guarantee 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.)

Each similar-origin window agent has a custom element reactions stack , which is initially empty. A similar-origin window agent 's current element queue is the element queue at the top of its custom element reactions stack . Each item in the stack is an element queue , which is initially empty as well. Each item in an element queue is an element. (The elements are not necessarily custom yet, since this queue is used for upgrades as well.)

Each custom element reactions stack has an associated backup element queue , which an initially-empty element queue . Elements are pushed onto the backup element queue during operations that affect the DOM without going through an API decorated with [CEReactions] , or through the parser's create an element for the token algorithm. An example of this is a user-initiated editing operation which modifies the descendants or attributes of an editable element. To prevent reentrancy when processing the backup element queue , each custom element reactions stack also has a processing the backup element queue flag, initially unset.

All elements have an associated custom element reaction queue , initially empty. Each item in the custom element reaction queue is of one of two types:

This is all summarized in the following schematic diagram:

A custom element reactions stack consists of a stack of element queues. Zooming in on a particular queue, we see that it contains a number of elements (in our example, <x-a>, then <x-b>, then <x-c> ). Any particular element in the queue then has a custom element reaction queue. Zooming in on the custom element reaction queue, we see that it contains a variety of queued-up reactions (in our example, upgrade, then attribute changed, then another attribute changed, then connected).

To enqueue an element on the appropriate element queue , given an element element , run the following steps:

  1. Let reactionsStack be element 's relevant agent 's custom element reactions stack .

  2. If reactionsStack is empty, then:

    1. Add element to reactionsStack 's backup element queue .

    2. If reactionsStack 's processing the backup element queue flag is set, then return.

    3. Set reactionsStack 's processing the backup element queue flag.

    4. Queue a microtask to perform the following steps:

      1. Invoke custom element reactions in reactionsStack 's backup element queue .

      2. Unset reactionsStack 's processing the backup element queue flag.

  3. Otherwise, add element to element 's relevant agent 's current element queue .

To enqueue a custom element callback reaction , given a custom element element , a callback name callbackName , and a list of arguments args , run the following steps:

  1. Let definition be element 's custom element definition .

  2. Let callback be the value of the entry in definition 's lifecycle callbacks with key callbackName .

  3. If callback is null, then return

  4. If callbackName is " attributeChangedCallback ", then:

    1. Let attributeName be the first element of args .

    2. If definition 's observed attributes does not contain attributeName , then return.

  5. Add a new callback reaction to element 's custom element reaction queue , with callback function callback and arguments args .

  6. Enqueue an element on the appropriate element queue given element .

To enqueue a custom element upgrade reaction , given an element element and custom element definition definition , run the following steps:

  1. Add a new upgrade reaction to element 's custom element reaction queue , with custom element definition definition .

  2. Enqueue an element on the appropriate element queue given element .

To invoke custom element reactions in an element queue queue , run the following steps:

  1. While queue is not empty :

    1. Let element be the result of dequeuing from queue .

    2. Let reactions be element 's custom element reaction queue .

    3. Repeat until reactions is empty:

      1. Remove the first element of reactions , and let reaction be that element. Switch on reaction 's type:

        upgrade reaction
        Upgrade element using reaction 's custom element definition .
        callback reaction
        Invoke reaction 's callback function with reaction 's arguments, and with element as the callback this value .

        If this throws an exception, catch it, and report the exception .

To ensure custom element reactions are triggered appropriately, we introduce the [CEReactions] IDL extended attribute . It indicates that the relevant algorithm is to be supplemented with additional steps in order to appropriately track and invoke custom element reactions .

The [CEReactions] extended attribute must take no arguments, and must not appear on anything other than an operation, attribute, setter, or deleter. Additionally, it must not appear on readonly attributes.

Operations, attributes, setters, or deleters annotated with the [CEReactions] extended attribute must run the following steps in place of the ones specified in their description:

  1. Push a new element queue onto this object's relevant agent 's custom element reactions stack .

  2. Run the originally-specified steps for this construct, catching any exceptions. If the steps return a value, let value be the returned value. If they throw an exception, let exception be the thrown exception.

  3. Let queue be the result of popping from this object's relevant agent 's custom element reactions stack .

  4. Invoke custom element reactions in queue .

  5. If an exception exception was thrown by the original steps, rethrow exception .

  6. If a value value was returned from the original steps, return value .

The intent behind this extended attribute is somewhat subtle. One way of accomplishing its goals would be to say that every operation, attribute, setter, and deleter on the platform must have these steps inserted, and to allow implementers to optimize away unnecessary cases (where no DOM mutation is possible that could cause custom element reactions to occur).

However, in practice this imprecision could lead to non-interoperable implementations of custom element reactions , as some implementations might forget to invoke these steps in some cases. Instead, we settled on the approach of explicitly annotating all relevant IDL constructs, as a way of ensuring interoperable behavior and helping implementations easily pinpoint all cases where these steps are necessary.

Any nonstandard APIs introduced by the user agent that could modify the DOM in such a way as to cause enqueuing a custom element callback reaction or enqueuing a custom element upgrade reaction , for example by modifying any attributes or child elements, must also be decorated with the [CEReactions] attribute.

As of the time of this writing, the following nonstandard or not-yet-standardized APIs are known to fall into this category:

4.13.7 Element internals

Certain capabilities are meant to be available to a custom element author, but not to a custom element consumer. These are provided by the element.attachInternals() method, which returns an instance of ElementInternals . The properties and methods of ElementInternals allow control over internal features which the user agent provides to all elements.

element . attachInternals()

Returns an ElementInternals object targeting the custom element element . Throws an exception if element is not a custom element , if the " internals " feature was disabled as part of the element definition, or if it is called twice on the same element.

Each HTMLElement has an attached internals boolean, initially false.

The attachInternals() method steps are:

  1. If this 's is value is not null, then throw a " NotSupportedError " DOMException .

  2. Let definition be the result of looking up a custom element definition given this's node document , its namespace, its local name, and null as the is value .

  3. If definition is null, then throw an " NotSupportedError " DOMException .

  4. If definition 's disable internals is true, then throw a " NotSupportedError " DOMException .

  5. If this 's attached internals is true, then throw an " NotSupportedError " DOMException .

  6. If this 's custom element state is not " precustomized " or " custom ", then throw a " NotSupportedError " DOMException .

  7. Set this 's attached internals to true.

  8. Return a new ElementInternals instance whose target element is this .

4.13.7.1 The ElementInternals interface

The IDL for the ElementInternals interface is as follows, with the various operations and attributes defined in the following sections: 

[Exposed=Window]
interface ElementInternals {
  // Shadow root access
  readonly attribute ShadowRoot? shadowRoot;
  // Form-associated custom elements
  undefined setFormValue((File or USVString or FormData)? value,
                         optional (File or USVString or FormData)? state);
  readonly attribute HTMLFormElement? form;
  undefined setValidity(optional ValidityStateFlags flags = {},
                        optional DOMString message,
                        optional HTMLElement anchor);
  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  boolean reportValidity();
  readonly attribute NodeList labels;
};
// Accessibility semantics
ElementInternals includes ARIAMixin;
dictionary ValidityStateFlags {
  boolean valueMissing = false;
  boolean typeMismatch = false;
  boolean patternMismatch = false;
  boolean tooLong = false;
  boolean tooShort = false;
  boolean rangeUnderflow = false;
  boolean rangeOverflow = false;
  boolean stepMismatch = false;
  boolean badInput = false;
  boolean customError = false;
};

Each ElementInternals has a target element , which is a custom element .

4.13.7.2 Shadow root access
internals . shadowRoot

Returns the ShadowRoot for internals 's target element , if the target element is a shadow host , or null otherwise.

The shadowRoot getter steps are:

  1. Let target be this 's target element .

  2. If target is not a shadow host , then return null.

  3. Let shadow be target 's shadow root .

  4. If shadow 's available to element internals is false, then return null.

  5. Return shadow .

4.13.7.3 Form-associated custom elements
internals . setFormValue ( value )

Sets both the state and submission value of internals 's target element to value .

If value is null, the element won't participate in form submission.

internals . setFormValue ( value , state )

Sets the submission value of internals 's target element to value , and its state to state .

If value is null, the element won't participate in form submission.

internals . form

Returns the form owner of internals 's target element .

internals . setValidity ( flags , message [, anchor ])

Marks internals 's target element as suffering from the constraints indicated by the flags argument, and sets the element's validation message to message . If anchor is specified, the user agent might use it to indicate problems with the constraints of internals 's target element when the form owner is validated interactively or reportValidity() is called.

internals . setValidity ({})

Marks internals 's target element as satisfying its constraints .

internals . willValidate

Returns true if internals 's target element will be validated when the form is submitted; false otherwise.

internals . validity

Returns the ValidityState object for internals 's target element .

internals . validationMessage

Returns the error message that would be shown to the user if internals 's target element was to be checked for validity.

valid = internals . checkValidity()

Returns true if internals 's target element has no validity problems; false otherwise. Fires an invalid event at the element in the latter case.

valid = internals . reportValidity()

Returns true if internals 's target element has no validity problems; otherwise, returns false, fires an invalid event at the element, and (if the event isn't canceled) reports the problem to the user.

internals . labels

Returns a NodeList of all the label elements that internals 's target element is associated with.

Each form-associated custom element has submission value . It is used to provide one or more entries on form submission. The initial value of submission value is null, and submission value can be null, a string, a File , or a list of entries .

Each form-associated custom element has state . It is information with which the user agent can restore a user's input for the element. The initial value of state is null, and state can be null, a string, a File , or a list of entries .

The setFormValue() method is used by the custom element author to set the element's submission value and state , thus communicating these to the user agent.

When the user agent believes it is a good idea to restore a form-associated custom element 's state , for example after navigation or restarting the user agent, they may enqueue a custom element callback reaction with that element, callback name " formStateRestoreCallback ", and an argument list containing the state to be restored, and "restore" .

If the user agent has a form-filling assist feature, then when the feature is invoked, it may enqueue a custom element callback reaction with a form-associated custom element , callback name " formStateRestoreCallback ", and an argument list containing the state value determined by history of state value and some heuristics, and "autocomplete" .

In general, the state is information specified by a user, and the submission value is a value after canonicalization or sanitization, suitable for submission to the server. The following examples makes this concrete:

Suppose that we have a form-associated custom element which asks a user to specify a date. The user specifies "3/15/2019" , but the control wishes to submit "2019-03-15" to the server. "3/15/2019" would be a state of the element, and "2019-03-15" would be a submission value .

Suppose you develop a custom element emulating a the behavior of the existing checkbox input type. Its submission value would be the value of its value content attribute, or the string "on" . Its state would be one of "checked" , "unchecked" , "checked/indeterminate" , or "unchecked/indeterminate" .

The setFormValue( value , state ) method steps are:

  1. Let element be this 's target element .

  2. If element is not a form-associated custom element , then throw a " NotSupportedError " DOMException .

  3. Set target element 's submission value to value if value is not a FormData object, or to a clone of the entry list associated with value otherwise.

  4. If the state argument of the function is omitted, set element 's state to its submission value .

  5. Otherwise, if state is a FormData object, set element 's state to clone of the entry list associated with state .

  6. Otherwise, set element 's state to state .

Each form-associated custom element has validity flags named valueMissing , typeMismatch , patternMismatch , tooLong , tooShort , rangeUnderflow , rangeOverflow , stepMismatch , and customError . They are false initially.

Each form-associated custom element has a validation message string. It is the empty string initially.

Each form-associated custom element has a validation anchor element. It is null initially.

The setValidity( flags , message , anchor ) method steps are:

  1. Let element be this 's target element .

  2. If element is not a form-associated custom element , then throw a " NotSupportedError " DOMException .

  3. If flags contains one or more true values and message is not given or is the empty string, then throw a TypeError .

  4. For each entry flag → value of flags , set element 's validity flag with the name flag to value .

  5. Set element 's validation message to the empty string if message is not given or all of element 's validity flags are false, or to message otherwise.

  6. If element 's customError validity flag is true, then set element 's custom validity error message to element 's validation message . Otherwise, set element 's custom validity error message to the empty string.

  7. Set element 's validation anchor to null if anchor is not given. Otherwise, if anchor is not a shadow-including descendant of element , then throw a " NotFoundError " DOMException . Otherwise, set element 's validation anchor to anchor .

The validationMessage getter steps are to return the validation message of this 's target element .

The entry construction algorithm for a form-associated custom element , given an element element and a list entry list , consists of the following steps:

  1. If element 's submission value is a list of entries , then append each item of element 's submission value to entry list , and return.

    In this case, user agent does not refer to the name content attribute value. An implementation of form-associated custom element is responsible to decide names of entries . They can be the name content attribute value, they can be strings based on the name content attribute value, or they can be unrelated to the name content attribute.

  2. If the element does not have a name attribute specified, or its name attribute's value is the empty string, then return.

  3. If the element's submission value is not null, append an entry to entry list with the name attribute value and the submission value .

4.13.7.4 Accessibility semantics
internals . role [ = value ]

Sets or retrieves the default ARIA role for internals 's target element , which will be used unless the page author overrides it using the role attribute.

internals . aria* [ = value ]

Sets or retrieves various default ARIA states or property values for internals 's target element , which will be used unless the page author overrides them using the aria-* attributes.

Each custom element has a native accessibility semantics map , which is a map , initially empty. See the Requirements related to ARIA and to platform accessibility APIs section for information on how this impacts platform accessibility APIs.

ElementInternals includes the ARIAMixin mixin. The accessors provided by this mixin are used to manipulate the target element 's native accessibility semantics map , as follows:

The ARIAMixin getter steps for ElementInternals , given internals , idlAttribute , and contentAttribute , are:

  1. Let map be internals 's target element 's native accessibility semantics map .

  2. If map [ contentAttribute ] exists , then return it.

  3. Return null.

The ARIAMixin setter steps for ElementInternals , given internals , idlAttribute , contentAttribute , and value , are:

  1. Let map be internals 's target element 's native accessibility semantics map .

  2. If value is null, then remove map [ contentAttribute ].

  3. Otherwise, set map [ contentAttribute ] to value .