HTML: The Living Standard

Edition for Web Developers — Last Updated 16 January 2024

1. 1. 4.12 Scripting
      1. 4.12.1 The script element
         1. 4.12.1.1 Scripting languages
         2. 4.12.1.2 Restrictions for contents of script elements
         3. 4.12.1.3 Inline documentation for external scripts
      2. 4.12.2 The noscript element
      3. 4.12.3 The template element
      4. 4.12.4 The slot element

4.12 Scripting

Scripts allow authors to add interactivity to their documents.

Authors are encouraged to use declarative alternatives to scripting where possible, as declarative mechanisms are often more maintainable, and many users disable scripting.

Authors are also encouraged to make their applications degrade gracefully in the absence of scripting support.

4.12.1 The script element

Categories:

Metadata content.

Flow content.

Phrasing content.

Script-supporting element.

Contexts in which this element can be used:

Where metadata content is expected.

Where phrasing content is expected.

Where script-supporting elements are expected.

Content model:

If there is no src attribute, depends on the value of the type attribute, but must match script content restrictions.

If there is a src attribute, the element must be either empty or contain only script documentation that also matches script content restrictions.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

src — Address of the resource

type — Type of script

nomodule — Prevents execution in user agents that support module scripts

async — Execute script when available, without blocking while fetching

defer — Defer script execution

crossorigin — How the element handles crossorigin requests

integrity — Integrity metadata used in Subresource Integrity checks [SRI]

referrerpolicy — Referrer policy for fetches initiated by the element

blocking — Whether the element is potentially render-blocking

fetchpriority — Sets the priority for fetches initiated by the element

Accessibility considerations:

For authors.

For implementers.

DOM interface:

Uses HTMLScriptElement.

The script element allows authors to include dynamic script and data blocks in their documents. The element does not represent content for the user.

The type attribute allows customization of the type of script represented:

• Omitting the attribute, setting it to the empty string, or setting it to a JavaScript MIME type essence match, means that the script is a classic script, to be interpreted according to the JavaScript Script top-level production. Classic scripts are affected by the async and defer attributes, but only when the src attribute is set. Authors should omit the type attribute instead of redundantly setting it.

• Setting the attribute to an ASCII case-insensitive match for "module" means that the script is a JavaScript module script, to be interpreted according to the JavaScript Module top-level production. Module scripts are not affected by the defer attribute, but are affected by the async attribute (regardless of the state of the src attribute).

• Setting the attribute to an ASCII case-insensitive match for "importmap" means that the script is an import map, containing JSON that will be used to control the behavior of module specifier resolution. Import maps can only be inline, i.e., the src attribute and most other attributes are meaningless and not to be used with them.

• Setting the attribute to any other value means that the script is a data block, which is not processed. None of the script attributes (except type itself) have any effect on data blocks. Authors must use a valid MIME type string that is not a JavaScript MIME type essence match to denote data blocks.

The requirement that data blocks must be denoted using a valid MIME type string is in place to avoid potential future collisions. If this specification ever adds additional types of script, they will be triggered by setting the type attribute to something which is not a MIME type, like how the "module" value denotes module scripts. By using a valid MIME type string now, you ensure that your data block will not ever be reinterpreted as a different script type, even in future user agents.

Classic scripts and JavaScript module scripts can be embedded inline, or be imported from an external file using the src attribute, which if specified gives the URL of the external script resource to use. If src is specified, it must be a valid non-empty URL potentially surrounded by spaces.

The contents of inline script elements, or the external script resource, must conform with the requirements of the JavaScript specification's Script or Module productions, for classic scripts and JavaScript module scripts respectively. [JAVASCRIPT]

The contents of the external script resource for CSS module scripts must conform to the requirements of the CSS specification. [CSS]

The contents of the external script resource for JSON module scripts must conform to the requirements of the JSON specification [JSON].

The contents of inline script elements for import maps must conform with the import map authoring requirements.

For import map script elements, the src, async, nomodule, defer, crossorigin, integrity, and referrerpolicy attributes must not be specified.

A document must not have more than one import map script element.

When used to include data blocks, the data must be embedded inline, the format of the data must be given using the type attribute, and the contents of the script element must conform to the requirements defined for the format used. The src, async, nomodule, defer, crossorigin, integrity, referrerpolicy, and fetchpriority attributes must not be specified.

The nomodule attribute is a boolean attribute that prevents a script from being executed in user agents that support module scripts. This allows selective execution of module scripts in modern user agents and classic scripts in older user agents, as shown below. The nomodule attribute must not be specified on module scripts (and will be ignored if it is).

The async and defer attributes are boolean attributes that indicate how the script should be evaluated. Classic scripts may specify defer or async, but must not specify either unless the src attribute is present. Module scripts may specify the async attribute, but must not specify the defer attribute.

There are several possible modes that can be selected using these attributes, and depending on the script's type.

For classic scripts, if the async attribute is present, then the classic script will be fetched in parallel to parsing and evaluated as soon as it is available (potentially before parsing completes). If the async attribute is not present but the defer attribute is present, then the classic script will be fetched in parallel and evaluated when the page has finished parsing. If neither attribute is present, then the script is fetched and evaluated immediately, blocking parsing until these are both complete.

For module scripts, if the async attribute is present, then the module script and all its dependencies will be fetched in parallel to parsing, and the module script will be evaluated as soon as it is available (potentially before parsing completes). Otherwise, the module script and its dependencies will be fetched in parallel to parsing and evaluated when the page has finished parsing. (The defer attribute has no effect on module scripts.)

This is all summarized in the following schematic diagram:

The exact processing details for these attributes are, for mostly historical reasons, somewhat non-trivial, involving a number of aspects of HTML. The implementation requirements are therefore by necessity scattered throughout the specification. The algorithms below (in this section) describe the core of this processing, but these algorithms reference and are referenced by the parsing rules for script start and end tags in HTML, in foreign content, and in XML, the rules for the document.write() method, the handling of scripting, etc.

When inserted using the document.write() method, script elements usually execute (typically blocking further script execution or HTML parsing). When inserted using the innerHTML and outerHTML attributes, they do not execute at all.

The defer attribute may be specified even if the async attribute is specified, to cause legacy web browsers that only support defer (and not async) to fall back to the defer behavior instead of the blocking behavior that is the default.

The crossorigin attribute is a CORS settings attribute. For classic scripts, it controls whether error information will be exposed, when the script is obtained from other origins. For module scripts, it controls the credentials mode used for cross-origin requests.

Unlike classic scripts, module scripts require the use of the CORS protocol for cross-origin fetching.

The integrity attribute represents the integrity metadata for requests which this element is responsible for. The value is text. The integrity attribute must not be specified when the src attribute is not specified. [SRI]

The referrerpolicy attribute is a referrer policy attribute. Its purpose is to set the referrer policy used when fetching the script, as well as any scripts imported from it. [REFERRERPOLICY]

<script referrerpolicy="origin">
  fetch('/api/data');    // not fetched with <script>'s referrer policy
  import('./utils.mjs'); // is fetched with <script>'s referrer policy ("origin" in this case)
</script>

The blocking attribute is a blocking attribute.

The fetchpriority attribute is a fetch priority attribute. Its purpose is to set the priority used when fetching the script.

Changing the src, type, nomodule, async, defer, crossorigin, integrity, referrerpolicy, and fetchpriority attributes dynamically has no direct effect; these attributes are only used at specific times described below.

script.text [ = value ]

Returns the child text content of the element.

Can be set, to replace the element's children with the given value.

HTMLScriptElement.supports(type)

✔MDN

HTMLScriptElement/supports_static

Support in all current engines.

Firefox94+Safari16+Chrome96+

---

Opera?Edge96+

---

Edge (Legacy)?Internet ExplorerNo

---

Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Returns true if the given type is a script type supported by the user agent. The possible script types in this specification are "classic", "module", and "importmap", but others might be added in the future.

The type argument has to exactly match these values; we do not perform an ASCII case-insensitive match. This is different from how type content attribute values are treated, and how DOMTokenList's supports() method works, but it aligns with the WorkerType enumeration used in the Worker() constructor.

<script src="game-engine.js"></script>
<script type="text/x-game-map">
........U.........e
o............A....e
.....A.....AAA....e
.A..AAA...AAAAA...e
</script>

<script>
 function calculate(form) {
   var price = 52000;
   if (form.elements.brakes.checked)
     price += 1000;
   if (form.elements.radio.checked)
     price += 2500;
   if (form.elements.turbo.checked)
     price += 5000;
   if (form.elements.sticker.checked)
     price += 250;
   form.elements.result.value = price;
 }
</script>
<form name="pricecalc" onsubmit="return false" onchange="calculate(this)">
 <fieldset>
  <legend>Work out the price of your car</legend>
  <p>Base cost: £52000.</p>
  <p>Select additional options:</p>
  <ul>
   <li><label><input type=checkbox name=brakes> Ceramic brakes (£1000)</label></li>
   <li><label><input type=checkbox name=radio> Satellite radio (£2500)</label></li>
   <li><label><input type=checkbox name=turbo> Turbo charger (£5000)</label></li>
   <li><label><input type=checkbox name=sticker> "XZ" sticker (£250)</label></li>
  </ul>
  <p>Total: £<output name=result></output></p>
 </fieldset>
 <script>
  calculate(document.forms.pricecalc);
 </script>
</form>

<script type="module" src="app.mjs"></script>

<script type="module" src="app.mjs"></script>
<script nomodule defer src="classic-app-bundle.js"></script>

<script type="module">
 import { walkAllTextNodeDescendants } from "./dom-utils.mjs";

 const substitutions = new Map([
   ["witnesses", "these dudes I know"]
   ["allegedly", "kinda probably"]
   ["new study", "Tumblr post"]
   ["rebuild", "avenge"]
   ["space", "spaaace"]
   ["Google glass", "Virtual Boy"]
   ["smartphone", "Pokédex"]
   ["electric", "atomic"]
   ["Senator", "Elf-Lord"]
   ["car", "cat"]
   ["election", "eating contest"]
   ["Congressional leaders", "river spirits"]
   ["homeland security", "Homestar Runner"]
   ["could not be reached for comment", "is guilty and everyone knows it"]
 ]);

 function substitute(textNode) {
   for (const [before, after] of substitutions.entries()) {
     textNode.data = textNode.data.replace(new RegExp(`\\b${before}\\b`, "ig"), after);
   }
 }

 walkAllTextNodeDescendants(document.body, substitute);
</script>

<script type="module">
 import peopleInSpace from "http://api.open-notify.org/astros.json" with { type: "json" };

 const list = document.querySelector("#people-in-space");
 for (const { craft, name } of peopleInSpace.people) {
   const li = document.createElement("li");
   li.textContent = `${name} / ${craft}`;
   list.append(li);
 }
</script>

4.12.1.1 Scripting languages

Servers should use text/javascript for JavaScript resources, in accordance with Updates to ECMAScript Media Types. Servers should not use other JavaScript MIME types for JavaScript resources, and must not use non-JavaScript MIME types. [RFC9239]

4.12.1.2 Restrictions for contents of script elements

The easiest and safest way to avoid the rather strange restrictions described in this section is to always escape an ASCII case-insensitive match for "<!--" as "\x3C!--", "<script" as "\x3Cscript", and "</script" as "\x3C/script" when these sequences appear in literals in scripts (e.g. in strings, regular expressions, or comments), and to avoid writing code that uses such constructs in expressions. Doing so avoids the pitfalls that the restrictions in this section are prone to triggering: namely, that, for historical reasons, parsing of script blocks in HTML is a strange and exotic practice that acts unintuitively in the face of these sequences.

The script element's descendant text content must match the script production in the following ABNF, the character set for which is Unicode. [ABNF]

script        = outer *( comment-open inner comment-close outer )

outer         = < any string that doesn't contain a substring that matches not-in-outer >
not-in-outer  = comment-open
inner         = < any string that doesn't contain a substring that matches not-in-inner >
not-in-inner  = comment-close / script-open

comment-open  = "<!--"
comment-close = "-->"
script-open   = "<" s c r i p t tag-end

s             =  %x0053 ; U+0053 LATIN CAPITAL LETTER S
s             =/ %x0073 ; U+0073 LATIN SMALL LETTER S
c             =  %x0043 ; U+0043 LATIN CAPITAL LETTER C
c             =/ %x0063 ; U+0063 LATIN SMALL LETTER C
r             =  %x0052 ; U+0052 LATIN CAPITAL LETTER R
r             =/ %x0072 ; U+0072 LATIN SMALL LETTER R
i             =  %x0049 ; U+0049 LATIN CAPITAL LETTER I
i             =/ %x0069 ; U+0069 LATIN SMALL LETTER I
p             =  %x0050 ; U+0050 LATIN CAPITAL LETTER P
p             =/ %x0070 ; U+0070 LATIN SMALL LETTER P
t             =  %x0054 ; U+0054 LATIN CAPITAL LETTER T
t             =/ %x0074 ; U+0074 LATIN SMALL LETTER T

tag-end       =  %x0009 ; U+0009 CHARACTER TABULATION (tab)
tag-end       =/ %x000A ; U+000A LINE FEED (LF)
tag-end       =/ %x000C ; U+000C FORM FEED (FF)
tag-end       =/ %x0020 ; U+0020 SPACE
tag-end       =/ %x002F ; U+002F SOLIDUS (/)
tag-end       =/ %x003E ; U+003E GREATER-THAN SIGN (>)

When a script element contains script documentation, there are further restrictions on the contents of the element, as described in the section below.

const example = 'Consider this string: <!-- <script>';
console.log(example);

<script>
  const example = 'Consider this string: <!-- <script>';
  console.log(example);
</script>

<script>
  const example = 'Consider this string: <!-- <script>';
  console.log(example);
</script>
<!-- despite appearances, this is actually part of the script still! -->
<script>
 ... // this is the same script block still...
</script>

<script>
  // Note: `\x3C` is an escape sequence for `<`.
  const example = 'Consider this string: \x3C!-- \x3Cscript>';
  console.log(example);
</script>
<!-- this is just a comment between script blocks -->
<script>
 ... // this is a new script block
</script>

if (x<!--y) { ... }
if ( player<script ) { ... }

if (x < !--y) { ... }
if (!--y > x) { ... }
if (!(--y) > x) { ... }
if (player < script) { ... }
if (script > player) { ... }

4.12.1.3 Inline documentation for external scripts

If a script element's src attribute is specified, then the contents of the script element, if any, must be such that the value of the text IDL attribute, which is derived from the element's contents, matches the documentation production in the following ABNF, the character set for which is Unicode. [ABNF]

documentation = *( *( space / tab / comment ) [ line-comment ] newline )
comment       = slash star *( not-star / star not-slash ) 1*star slash
line-comment  = slash slash *not-newline

; characters
tab           = %x0009 ; U+0009 CHARACTER TABULATION (tab)
newline       = %x000A ; U+000A LINE FEED (LF)
space         = %x0020 ; U+0020 SPACE
star          = %x002A ; U+002A ASTERISK (*)
slash         = %x002F ; U+002F SOLIDUS (/)
not-newline   = %x0000-0009 / %x000B-10FFFF
                ; a scalar value other than U+000A LINE FEED (LF)
not-star      = %x0000-0029 / %x002B-10FFFF
                ; a scalar value other than U+002A ASTERISK (*)
not-slash     = %x0000-002E / %x0030-10FFFF
                ; a scalar value other than U+002F SOLIDUS (/)

This corresponds to putting the contents of the element in JavaScript comments.

This requirement is in addition to the earlier restrictions on the syntax of contents of script elements.

<script src="cool-effects.js">
 // create new instances using:
 //    var e = new Effect();
 // start the effect using .play, stop using .stop:
 //    e.play();
 //    e.stop();
</script>

4.12.2 The noscript element

Categories:

Metadata content.

Flow content.

Phrasing content.

Contexts in which this element can be used:

In a head element of an HTML document, if there are no ancestor noscript elements.

Where phrasing content is expected in HTML documents, if there are no ancestor noscript elements.

Content model:

When scripting is disabled, in a head element: in any order, zero or more link elements, zero or more style elements, and zero or more meta elements.

When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.

Otherwise: text that conforms to the requirements given in the prose.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

Accessibility considerations:

For authors.

For implementers.

DOM interface:

Uses HTMLElement.

The noscript element represents nothing if scripting is enabled, and represents its children if scripting is disabled. It is used to present different markup to user agents that support scripting and those that don't support scripting, by affecting how the document is parsed.

When used in HTML documents, the allowed content model is as follows:

In a head element, if scripting is disabled for the noscript element

The noscript element must contain only link, style, and meta elements.

In a head element, if scripting is enabled for the noscript element

The noscript element must contain only text, except that invoking the HTML fragment parsing algorithm with the noscript element as the context element and the text contents as the input must result in a list of nodes that consists only of link, style, and meta elements that would be conforming if they were children of the noscript element, and no parse errors.

Outside of head elements, if scripting is disabled for the noscript element

The noscript element's content model is transparent, with the additional restriction that a noscript element must not have a noscript element as an ancestor (that is, noscript can't be nested).

Outside of head elements, if scripting is enabled for the noscript element

The noscript element must contain only text, except that the text must be such that running the following algorithm results in a conforming document with no noscript elements and no script elements, and such that no step in the algorithm throws an exception or causes an HTML parser to flag a parse error:

1. Remove every script element from the document.
2. Make a list of every noscript element in the document. For every noscript element in that list, perform the following steps:
   1. Let s be the child text content of the noscript element.
   2. Set the outerHTML attribute of the noscript element to the value of s. (This, as a side-effect, causes the noscript element to be removed from the document.) [DOMPARSING]

All these contortions are required because, for historical reasons, the noscript element is handled differently by the HTML parser based on whether scripting was enabled or not when the parser was invoked.

The noscript element must not be used in XML documents.

The noscript element is only effective in the HTML syntax, it has no effect in the XML syntax. This is because the way it works is by essentially "turning off" the parser when scripts are enabled, so that the contents of the element are treated as pure text and not as real elements. XML does not define a mechanism by which to do this.

<form action="calcSquare.php">
 <p>
  <label for=x>Number</label>:
  <input id="x" name="x" type="number">
 </p>
 <script>
  var x = document.getElementById('x');
  var output = document.createElement('p');
  output.textContent = 'Type a number; it will be squared right then!';
  x.form.appendChild(output);
  x.form.onsubmit = function () { return false; }
  x.oninput = function () {
    var v = x.valueAsNumber;
    output.textContent = v + ' squared is ' + v * v;
  };
 </script>
 <noscript>
  <input type=submit value="Calculate Square">
 </noscript>
</form>

<form action="calcSquare.php">
 <p>
  <label for=x>Number</label>:
  <input id="x" name="x" type="number">
 </p>
 <input id="submit" type=submit value="Calculate Square">
 <script>
  var x = document.getElementById('x');
  var output = document.createElement('p');
  output.textContent = 'Type a number; it will be squared right then!';
  x.form.appendChild(output);
  x.form.onsubmit = function () { return false; }
  x.oninput = function () {
    var v = x.valueAsNumber;
    output.textContent = v + ' squared is ' + v * v;
  };
  var submit = document.getElementById('submit');
  submit.parentNode.removeChild(submit);
 </script>
</form>

4.12.3 The template element

Categories:

Metadata content.

Flow content.

Phrasing content.

Script-supporting element.

Contexts in which this element can be used:

Where metadata content is expected.

Where phrasing content is expected.

Where script-supporting elements are expected.

As a child of a colgroup element that doesn't have a span attribute.

Content model:

Nothing (for clarification, see example).

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

shadowrootmode — Enables streaming declarative shadow roots

shadowrootdelegatesfocus — Sets delegates focus on a declarative shadow root

Accessibility considerations:

For authors.

For implementers.

DOM interface:

Uses HTMLTemplateElement.

The template element is used to declare fragments of HTML that can be cloned and inserted in the document by script.

In a rendering, the template element represents nothing.

The shadowrootmode content attribute is an enumerated attribute with the following keywords and states:

The shadowrootmode attribute's invalid value default and missing value default are both the none state.

The shadowrootdelegatesfocus content attribute is a boolean attribute.

The template contents of a template element are not children of the element itself.

It is also possible, as a result of DOM manipulation, for a template element to contain Text nodes and element nodes; however, having any is a violation of the template element's content model, since its content model is defined as nothing.

<!doctype html>
<html lang="en">
 <head>
  <title>Homework</title>
 <body>
  <template id="template"><p>Smile!</p></template>
  <script>
   let num = 3;
   const fragment = document.getElementById('template').content.cloneNode(true);
   while (num-- > 1) {
     fragment.firstChild.before(fragment.firstChild.cloneNode(true));
     fragment.firstChild.textContent += fragment.lastChild.textContent;
   }
   document.body.appendChild(fragment);
  </script>
</html>

template.content

Returns the template contents (a DocumentFragment).

<!DOCTYPE html>
<html lang='en'>
<title>Cat data</title>
<script>
 // Data is hard-coded here, but could come from the server
 var data = [
   { name: 'Pillar', color: 'Ticked Tabby', sex: 'Female (neutered)', legs: 3 },
   { name: 'Hedral', color: 'Tuxedo', sex: 'Male (neutered)', legs: 4 },
 ];
</script>
<table>
 <thead>
  <tr>
   <th>Name <th>Color <th>Sex <th>Legs
 <tbody>
  <template id="row">
   <tr><td><td><td><td>
  </template>
</table>
<script>
 var template = document.querySelector('#row');
 for (var i = 0; i < data.length; i += 1) {
   var cat = data[i];
   var clone = template.content.cloneNode(true);
   var cells = clone.querySelectorAll('td');
   cells[0].textContent = cat.name;
   cells[1].textContent = cat.color;
   cells[2].textContent = cat.sex;
   cells[3].textContent = cat.legs;
   template.parentNode.appendChild(clone);
 }
</script>

4.12.4 The slot element

Categories:

Flow content.

Phrasing content.

Contexts in which this element can be used:

Where phrasing content is expected.

Content model:

Transparent

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

name — Name of shadow tree slot

Accessibility considerations:

For authors.

For implementers.

DOM interface:

Uses HTMLSlotElement.

The slot element defines a slot. It is typically used in a shadow tree. A slot element represents its assigned nodes, if any, and its contents otherwise.

The name content attribute may contain any string value. It represents a slot's name.

The name attribute is used to assign slots to other elements: a slot element with a name attribute creates a named slot to which any element is assigned if that element has a slot attribute whose value matches that name attribute's value, and the slot element is a child of the shadow tree whose root's host has that corresponding slot attribute value.

slot.name

Can be used to get and set slot's name.

slot.assignedNodes()

Returns slot's assigned nodes.

slot.assignedNodes({ flatten: true })

Returns slot's assigned nodes, if any, and slot's children otherwise, and does the same for any slot elements encountered therein, recursively, until there are no slot elements left.

slot.assignedElements()

Returns slot's assigned nodes, limited to elements.

slot.assignedElements({ flatten: true })

Returns the same as assignedNodes({ flatten: true }), limited to elements.

slot.assign(...nodes)

Sets slot's manually assigned nodes to the given nodes.

The name IDL attribute must reflect the content attribute of the same name.

The slot element has manually assigned nodes, which is an ordered set of slottables set by assign(). This set is initially empty.

The manually assigned nodes set can be implemented using weak references to the slottables, because this set is not directly accessible from script.

The assignedNodes(options) method steps are:

1. If options["flatten"] is false, then return this's assigned nodes.

2. Return the result of finding flattened slottables with this.

The assignedElements(options) method steps are:

1. If options["flatten"] is false, then return this's assigned nodes, filtered to contain only Element nodes.

2. Return the result of finding flattened slottables with this, filtered to contain only Element nodes.

The assign(...nodes) method steps are:

1. For each node of this's manually assigned nodes, set node's manual slot assignment to null.

2. Let nodesSet be a new ordered set.

3. For each node of nodes:

   1. If node's manual slot assignment refers to a slot, then remove node from that slot's manually assigned nodes.

   2. Set node's manual slot assignment to this.

   3. Append node to nodesSet.

4. Set this's manually assigned nodes to nodesSet.

5. Run assign slottables for a tree for this's root.