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 Custom element reactions
      6. 4.13.6 The ElementInternals interface

4.13 Custom elements

Using_custom_elements

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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.

4.13.1.1 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() {
    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

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

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 overriden 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

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

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 MouseEvent("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

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

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  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:

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 — Associates the element with a form element
disabled, for form-associated custom elements — Whether the form control is disabled
readonly, for form-associated custom elements — Affects willValidate, plus any behavior added by the custom element author
name, for form-associated custom elements — Name of the element to use for form submission and in the form.elements API
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-😍>.

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)

CustomElementRegistry/define

Support in all current engines.

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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)

CustomElementRegistry/get

Support in all current engines.

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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)

CustomElementRegistry/whenDefined

Support in all current engines.

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Firefox Android63+Safari iOS10.3+Chrome Android66+WebView Android66+Samsung Internet9.0+Opera Android47+
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)

CustomElementRegistry/upgrade

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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;

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;
  });

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 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.)

4.13.6 The ElementInternals interface

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.

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.

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 ElementInternals has a target element, which is a custom element. ElementInternals provides various operations and attributes to allow control over internal features which the user agent provides to all elements.

Each HTMLElement has an attached internals boolean, initially false.

The attachInternals() method on an HTMLElement element, when invoked, must run the following steps:

  1. If element'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 element's node document, its namespace, its local name, and null as 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 element's attached internals is true, then throw an "NotSupportedError" DOMException.

  6. Set element's attached internals to true.

  7. Create a new ElementInternals instance targeting element, and return it.

Each form-associated custom element has submission value. It is used to provide one or more entries on form submission, and 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".

By using the role and aria* properties of ElementInternals, custom element authors can set default accessibile roles, states, and property values for their custom element, similar to how native elements behave. See the example above for more details.