HTML: The Living Standard

Edition for Web Developers — Last Updated 9 June 2025

1. 8 Web application APIs
   1. 8.1 Scripting
      1. 8.1.1 Introduction
      2. 8.1.2 Agents and agent clusters
         1. 8.1.2.1 Integration with the JavaScript agent formalism
         2. 8.1.2.2 Integration with the JavaScript agent cluster formalism
      3. 8.1.3 Script processing model
         1. 8.1.3.1 Runtime script errors
         2. 8.1.3.2 Unhandled promise rejections
      4. 8.1.4 Module specifier resolution
         1. 8.1.4.1 The resolution algorithm
         2. 8.1.4.2 Import maps
         3. 8.1.4.3 Module-related host hooks
      5. 8.1.5 Event loops
      6. 8.1.6 Events
         1. 8.1.6.1 Event handlers
         2. 8.1.6.2 Event handlers on elements, Document objects, and Window objects
   2. 8.2 The WindowOrWorkerGlobalScope mixin
   3. 8.3 Base64 utility methods

8 Web application APIs

8.1 Scripting

8.1.1 Introduction

Various mechanisms can cause author-provided executable code to run in the context of a document. These mechanisms include, but are probably not limited to:

• Processing of script elements.
• Navigating to javascript: URLs.
• Event handlers, whether registered through the DOM using addEventListener(), by explicit event handler content attributes, by event handler IDL attributes, or otherwise.
• Processing of technologies like SVG that have their own scripting features.

8.1.2 Agents and agent clusters

8.1.2.1 Integration with the JavaScript agent formalism

JavaScript defines the concept of an agent. This section gives the mapping of that language-level concept on to the web platform.

The following types of agents exist on the web platform:

Similar-origin window agent

Contains various Window objects which can potentially reach each other, either directly or by using document.domain.

If the encompassing agent cluster's is origin-keyed is true, then all the Window objects will be same origin, can reach each other directly, and document.domain will no-op.

Two Window objects that are same origin can be in different similar-origin window agents, for instance if they are each in their own browsing context group.

Dedicated worker agent

Contains a single DedicatedWorkerGlobalScope.

Shared worker agent

Contains a single SharedWorkerGlobalScope.

Service worker agent

Contains a single ServiceWorkerGlobalScope.

Worklet agent

Contains a single WorkletGlobalScope object.

Although a given worklet can have multiple realms, each such realm needs its own agent, as each realm can be executing code independently and at the same time as the others.

Only shared and dedicated worker agents allow the use of JavaScript Atomics APIs to potentially block.

8.1.2.2 Integration with the JavaScript agent cluster formalism

JavaScript also defines the concept of an agent cluster, which this standard maps to the web platform by placing agents appropriately when they are created.

The agent cluster concept is crucial for defining the JavaScript memory model, and in particular among which agents the backing data of SharedArrayBuffer objects can be shared.

Conceptually, the agent cluster concept is an architecture-independent, idealized "process boundary" that groups together multiple "threads" (agents). The agent clusters defined by the specification are generally more restrictive than the actual process boundaries implemented in user agents. By enforcing these idealized divisions at the specification level, we ensure that web developers see interoperable behavior with regard to shared memory, even in the face of varying and changing user agent process models.

8.1.3 Script processing model

8.1.3.1 Runtime script errors

self.reportError(e)

Dispatches an error event at the global object for the given value e, in the same fashion as an unhandled exception.

In various scenarios, the user agent can report an exception by firing an error event at the Window. If this event is not canceled, then the error is considered not handled, and can be reported to the developer console.

8.1.3.2 Unhandled promise rejections

In addition to synchronous runtime script errors, scripts may experience asynchronous promise rejections, tracked via the unhandledrejection and rejectionhandled events.

8.1.4 Module specifier resolution

8.1.4.1 The resolution algorithm

The resolve a module specifier algorithm is the primary entry point for converting module specifier strings into URLs. When no import maps are involved, it is relatively straightforward, and reduces to resolving a URL-like module specifier.

When there is a non-empty import map present, the behavior is more complex. It checks candidate entries from all applicable module specifier maps, from most-specific to least-specific scopes (falling back to the top-level unscoped imports), and from most-specific to least-specific prefixes.

In the end, if no successful resolution is found via any of the candidate module specifier maps, resolve a module specifier will throw an exception. Thus the result is always either a URL or a thrown exception.

8.1.4.2 Import maps

An import map allows control over module specifier resolution. Import maps are delivered via inline script elements with their type attribute set to "importmap", and with their child text content containing a JSON representation of the import map.

A Document can have multiple import maps processed, which can happen either before or after any modules have been imported, e.g., via import() expressions or script elements with their type attribute set to "module". The merge existing and new import maps algorithm ensures that new import maps cannot define the module resolution for modules that were already defined by past import maps, or for ones that were already resolved.

{
  "imports": {
    "moment": "/node_modules/moment/src/moment.js"
  }
}

{
  "imports": {
    "moment/": "/node_modules/moment/src/"
  }
}

{
  "imports": {
    "moment": "/node_modules/moment/src/moment.js",
    "moment/": "/node_modules/moment/src/"
  }
}

{
  "imports": {
    "https://cdn.example.com/vue/dist/vue.runtime.esm.js": "/node_modules/vue/dist/vue.runtime.esm.js",
    "/js/app.mjs": "/js/app-8e0d62a03.mjs",
    "../helpers/": "https://cdn.example/helpers/"
  }
}

{
  "scopes": {
    "/a/" : {
      "moment": "/node_modules/moment/src/moment.js"
    },
    "/b/" : {
      "moment": "https://cdn.example.com/moment/src/moment.js"
    }
  }
}

{
  "imports": {
    "a": "/a-1.mjs",
    "b": "/b-1.mjs",
    "c": "/c-1.mjs"
  },
  "scopes": {
    "/scope2/": {
      "a": "/a-2.mjs"
    },
    "/scope2/scope3/": {
      "b": "/b-3.mjs"
    }
  }
}

{
  "imports": {
    "a": "/a-1.mjs",
    "b": "/b-1.mjs",
    "c": "/c-1.mjs"
  },
  "integrity": {
    "/a-1.mjs": "sha384-Li9vy3DqF8tnTXuiaAJuML3ky+er10rcgNR/VqsVpcw+ThHmYcwiB1pbOxEbzJr7",
    "/d-1.mjs": "sha384-MBO5IDfYaE6c6Aao94oZrIOiC6CGiSN2n4QUbHNPhzk5Xhm0djZLQqTpL0HzTUxk"
  }
}

The child text content of a script element representing an import map must match the following import map authoring requirements:

• It must be valid JSON. [JSON]

• The JSON must represent a JSON object, with at most the three keys "imports", "scopes", and "integrity".

• The values corresponding to the "imports", "scopes", and "integrity" keys, if present, must themselves be JSON objects.

• The value corresponding to the "imports" key, if present, must be a valid module specifier map.

• The value corresponding to the "scopes" key, if present, must be a JSON object, whose keys are valid URL strings and whose values are valid module specifier maps.

• The value corresponding to the "integrity" key, if present, must be a JSON object, whose keys are valid URL strings and whose values fit the requirements of the integrity attribute.

A valid module specifier map is a JSON object that meets the following requirements:

• All of its keys must be nonempty.

• All of its values must be strings.

• Each value must be either a valid absolute URL or a valid URL string that starts with "/", "./", or "../".

• If a given key ends with "/", then the corresponding value must also.

8.1.4.3 Module-related host hooks

The JavaScript specification defines a syntax for modules, as well as some host-agnostic parts of their processing model. This specification defines the rest of their processing model: how the module system is bootstrapped, via the script element with type attribute set to "module", and how modules are fetched, resolved, and executed. [JAVASCRIPT]

Although the JavaScript specification speaks in terms of "scripts" versus "modules", in general this specification speaks in terms of classic scripts versus module scripts, since both of them use the script element.

modulePromise = import(specifier)

Returns a promise for the module namespace object for the module script identified by specifier. This allows dynamic importing of module scripts at runtime, instead of statically using the import statement form. The specifier will be resolved relative to the active script.

The returned promise will be rejected if an invalid specifier is given, or if a failure is encountered while fetching or evaluating the resulting module graph.

This syntax can be used inside both classic and module scripts. It thus provides a bridge into the module-script world, from the classic-script world.

url = import.meta.url

Returns the active module script's base URL.

This syntax can only be used inside module scripts.

url = import.meta.resolve(specifier)

Returns specifier, resolved relative to the active script. That is, this returns the URL that would be imported by using import(specifier).

Throws a TypeError exception if an invalid specifier is given.

This syntax can only be used inside module scripts.

Module maps are used to ensure that imported module scripts are only fetched, parsed, and evaluated once per Document or worker.

import "https://example.com/module.mjs";
import "https://example.com/module.mjs#map-buster";
import "https://example.com/module.mjs?debug=true";

import "https://example.com/module2.mjs";
import "https:example.com/module2.mjs";
import "https://///example.com\\module2.mjs";
import "https://example.com/foo/../module2.mjs";

<script type=module>
  import "https://example.com/module";
</script>
<script type=module>
  import "https://example.com/module" with { type: "css" };
</script>

<script type="module">
    // All of the following will fail, assuming that the imported .mjs files are served with a
    // JavaScript MIME type. JavaScript module scripts are the default and cannot be imported with
    // any import type attribute.
    import foo from "./foo.mjs" with { type: "javascript" };
    import foo2 from "./foo2.mjs" with { type: "js" };
    import foo3 from "./foo3.mjs" with { type: "" };
    await import("./foo4.mjs", { with: { type: null } });
    await import("./foo5.mjs", { with: { type: undefined } });
</script>

8.1.5 Event loops

To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section. Each agent has an associated event loop, which is unique to that agent.

The event loop of a similar-origin window agent is known as a window event loop. The event loop of a dedicated worker agent, shared worker agent, or service worker agent is known as a worker event loop. And the event loop of a worklet agent is known as a worklet event loop.

8.1.6 Events

8.1.6.1 Event handlers

Many objects can have event handlers specified. These act as non-capture event listeners for the object on which they are specified. [DOM]

Event handlers are exposed in two ways.

The first way, common to all event handlers, is as an event handler IDL attribute.

The second way is as an event handler content attribute. Event handlers on HTML elements and some of the event handlers on Window objects are exposed in this way.

For both of these two ways, the event handler is exposed through a name, which is a string that always starts with "on" and is followed by the name of the event for which the handler is intended.

Most of the time, the object that exposes an event handler is the same as the object on which the corresponding event listener is added. However, the body and frameset elements expose several event handlers that act upon the element's Window object, if one exists. In either case, we call the object an event handler acts upon the target of that event handler.

An event handler IDL attribute is an IDL attribute for a specific event handler. The name of the IDL attribute is the same as the name of the event handler.

An event handler content attribute is a content attribute for a specific event handler. The name of the content attribute is the same as the name of the event handler.

Event handler content attributes, when specified, must contain valid JavaScript code which, when parsed, would match the FunctionBody production after automatic semicolon insertion.

<button id="test">Start Demo</button>
<script>
 var button = document.getElementById('test');
 button.addEventListener('click', function () { alert('ONE') }, false);
 button.setAttribute('onclick', "alert('NOT CALLED')"); // event handler listener is registered here
 button.addEventListener('click', function () { alert('THREE') }, false);
 button.onclick = function () { alert('TWO'); };
 button.addEventListener('click', function () { alert('FOUR') }, false);
</script>

<button id="test">Start Demo</button>
<script>
 var button = document.getElementById('test');
 button.addEventListener('click', function () { alert('ONE') }, false);
 button.setAttribute('onclick', "alert('NOT CALLED')"); // event handler is activated here
 button.addEventListener('click', function () { alert('TWO') }, false);
 button.onclick = null;                                 // but deactivated here
 button.addEventListener('click', function () { alert('THREE') }, false);
 button.onclick = function () { alert('FOUR'); };       // and re-activated here
 button.addEventListener('click', function () { alert('FIVE') }, false);
</script>

The EventHandler callback function type represents a callback used for event handlers.

In JavaScript, any Function object implements this interface.

<body onload="alert(this)" onclick="alert(this)">

For historical reasons, the onerror handler has different arguments:

window.onerror = (message, source, lineno, colno, error) => { … };

Similarly, the onbeforeunload handler has a different return value: it will be cast to a string.

8.1.6.2 Event handlers on elements, Document objects, and Window objects

The following are the event handlers (and their corresponding event handler event types) supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and supported by all Document and Window objects, as event handler IDL attributes:

The following are the event handlers (and their corresponding event handler event types) supported by all HTML elements other than body and frameset elements, as both event handler content attributes and event handler IDL attributes; supported by all Document objects, as event handler IDL attributes; and supported by all Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's associated Document:

We call the set of the names of the event handlers listed in the first column of this table the Window-reflecting body element event handler set.

The following are the event handlers (and their corresponding event handler event types) supported by Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's associated Document:

The following are the event handlers (and their corresponding event handler event types) supported on Document objects as event handler IDL attributes:

8.2 The WindowOrWorkerGlobalScope mixin

The WindowOrWorkerGlobalScope mixin is for use of APIs that are to be exposed on Window and WorkerGlobalScope objects.

Other standards are encouraged to further extend it using partial interface mixin WindowOrWorkerGlobalScope { … }; along with an appropriate reference.

self.isSecureContext

Returns whether or not this global object represents a secure context. [SECURE-CONTEXTS]

self.origin

Returns the global object's origin, serialized as string.

self.crossOriginIsolated

Returns whether scripts running in this global are allowed to use APIs that require cross-origin isolation. This depends on the `Cross-Origin-Opener-Policy` and `Cross-Origin-Embedder-Policy` HTTP response headers and the "cross-origin-isolated" feature.

var frame = document.createElement("iframe")
frame.onload = function() {
  var frameWin = frame.contentWindow
  console.log(frameWin.location.origin) // "null"
  console.log(frameWin.origin) // "https://stargate.example"
}
document.body.appendChild(frame)

8.3 Base64 utility methods

The atob() and btoa() methods allow developers to transform content to and from the base64 encoding.

In these APIs, for mnemonic purposes, the "b" can be considered to stand for "binary", and the "a" for "ASCII". In practice, though, for primarily historical reasons, both the input and output of these functions are Unicode strings.

result = self.btoa(data)

Takes the input data, in the form of a Unicode string containing only characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, and converts it to its base64 representation, which it returns.

Throws an "InvalidCharacterError" DOMException if the input string contains any out-of-range characters.

result = self.atob(data)

Takes the input data, in the form of a Unicode string containing base64-encoded binary data, decodes it, and returns a string consisting of characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, corresponding to that binary data.

Throws an "InvalidCharacterError" DOMException if the input string is not valid base64 data.