W3C

HTML 5.1 2nd Edition

W3C Recommendation

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.

For example, instead of using script to show or hide a section to show more details, the details element could be used.

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

For example, if an author provides a link in a table header to dynamically resort the table, the link could also be made to function without scripts by requesting the sorted table from the server.

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 embedded resource
charset - Character encoding of the external script resource
async - Execute script in parallel
defer - Defer script execution
crossorigin - How the element handles crossorigin requests
nonce - Cryptographic nonce used in Content Security Policy checks [CSP3]
Allowed ARIA role attribute values:
None
Allowed ARIA state and property attributes:
Global aria-* attributes
DOM interface:
interface HTMLScriptElement : HTMLElement {
  attribute DOMString src;
  attribute DOMString type;
  attribute DOMString charset;
  attribute boolean async;
  attribute boolean defer;
  attribute DOMString? crossOrigin;
  attribute DOMString text;
  attribute DOMString nonce;
};

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:

The requirement that data blocks must be denoted using a valid MIME type 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. By using a valid MIME type now, you ensure that your data block will not ever be reinterpreted as a different script type, even in future user agents.

Classic scripts may either be embedded inline or may 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 production for classic scripts. [ECMA-262]

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, charset, async, defer, crossorigin, and nonce attributes must not be specified.

The charset attribute gives the character encoding of the external script resource. The attribute must not be specified if the src attribute is not present, or if the script is not a classic script. If the attribute is set, its value must be an ASCII case-insensitive match for one of the labels of an encoding, and must specify the same encoding as the charset parameter of the Content-Type metadata of the external file, if any. [ENCODING]

The async and defer attributes are boolean attributes that indicate how the script should be executed. Classic scripts may specify defer or async.

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.

This is all summarized in the following schematic diagram:

With <script>, parsing is interrupted by fetching and execution. With <script defer>, fetching is parallel to parsing and execution takes place after all parsing has finished. And with <script async>, fetching is parallel to parsing but once it finishes parsing is interrupted to execute the script.

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.

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.

The nonce attribute represents a cryptographic nonce ("number used once") which can be used by Content Security Policy to determine whether or not the script specified by an element will be executed. The value is text. [CSP3]

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

The IDL attributes src, type, charset, defer, and nonce, must each reflect the respective content attributes of the same name.

The crossOrigin IDL attribute must reflect the crossorigin content attribute.

The async IDL attribute controls whether the element will execute in parallel or not. If the element’s "non-blocking" flag is set, then, on getting, the async IDL attribute must return true, and on setting, the "non-blocking" flag must first be unset, and then the content attribute must be removed if the IDL attribute’s new value is false, and must be set to the empty string if the IDL attribute’s new value is true. If the element’s "non-blocking" flag is not set, the IDL attribute must reflect the async content attribute.

script . text [ = value ]
Returns the child text content of the element.

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

The IDL attribute text must return the child text content of the script element. On setting, it must act the same way as the textContent IDL attribute.

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

In this example, two script elements are used. One embeds an external classic script, and the other includes some data as a data block.
<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>

The data in this case might be used by the script to generate the map of a video game. The data doesn’t have to be used that way, though; maybe the map data is actually embedded in other parts of the page’s markup, and the data block here is just used by the site’s search engine to help users who are looking for particular features in their game maps.

The following sample shows how a script element can be used to define a function that is then used by other parts of the document, as part of a classic script. It also shows how a script element can be used to invoke script while the document is being parsed, in this case to initialize the form’s output.
<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>
4.12.1.1. Processing model

A script element has several associated pieces of state.

The first is a flag indicating whether or not the script block has been "already started". Initially, script elements must have this flag unset (script blocks, when created, are not "already started"). The cloning steps for script elements must set the "already started" flag on the copy if it is set on the element being cloned.

The second is a flag indicating whether the element was "parser-inserted". Initially, script elements must have this flag unset. It is set by the HTML parser and the XML parser on script elements they insert and affects the processing of those elements.

The third is a flag indicating whether the element will "non-blocking". Initially, script elements must have this flag set. It is unset by the HTML parser and the XML parser on script elements they insert. In addition, whenever a script element whose "non-blocking" flag is set has an async content attribute added, the element’s "non-blocking" flag must be unset.

The fourth is a flag indicating whether or not the script block is "ready to be parser-executed". Initially, script elements must have this flag unset (script blocks, when created, are not "ready to be parser-executed"). This flag is used only for elements that are also "parser-inserted", to let the parser know when to execute the script.

The fifth is the script’s type, which is "classic". It is determined when the script is prepared, based on the type attribute of the element at that time. Initially, script elements must have this flag unset.

The sixth is a flag indicating whether or not the script is from an external file. It is determined when the script is prepared, based on the src attribute of the element at that time.

Finally, a script element has the script’s script, which is a script resulting from preparing the element. This is set asynchronously after the classic script is fetched. Once it is set, either to a script in the case of success or to null in the case of failure, the fetching algorithms will note that the script is ready, which can trigger other actions. The user agent must delay the load event of the element’s node document until the script is ready.

When a script element that is not marked as being "parser-inserted" experiences one of the events listed in the following list, the user agent must immediately prepare the script element:

To prepare a script, the user agent must act as follows:

  1. If the script element is marked as having "already started", then the user agent must abort these steps at this point. The script is not executed.

  2. If the element has its "parser-inserted" flag set, then set was-parser-inserted to true and unset the element’s "parser-inserted" flag. Otherwise, set was-parser-inserted to false.

    This is done so that if parser-inserted script elements fail to run when the parser tries to run them, e.g., because they are empty or specify an unsupported scripting language, another script can later mutate them and cause them to run again.

  3. If was-parser-inserted is true and the element does not have an async attribute, then set the element’s "non-blocking" flag to true.

    This is done so that if a parser-inserted script element fails to run when the parser tries to run it, but it is later executed after a script dynamically updates it, it will execute in a non-blocking fashion even if the async attribute isn’t set.

  4. If the element has no src attribute, and its child nodes, if any, consist only of comment nodes and empty Text nodes, then abort these steps at this point. The script is not executed.

  5. If the element is not in a Document, then the user agent must abort these steps at this point. The script is not executed.

  6. If either:

    • the script element has a type attribute and its value is the empty string, or

    • the script element has no type attribute but it has a language attribute and that attribute’s value is the empty string, or

    • the script element has neither a type attribute nor a language attribute, then

    ...let the script block’s type string for this script element be "`text/javascript`".

    Otherwise, if the script element has a type attribute, let the script block’s type string for this script element be the value of that attribute with any leading or trailing sequences of space characters removed.

    Otherwise, the element has a non-empty language attribute; let the script block’s type string for this script element be the concatenation of the string "`text/`" followed by the value of the language attribute.

    The language attribute is never conforming, and is always ignored if there is a type attribute present.

    Determine the script’s type as follows:

  7. If was-parser-inserted is true, then flag the element as "parser-inserted" again, and set the element’s "non-blocking" flag to false.

  8. The user agent must set the element’s "already started" flag.

  9. If the element is flagged as "parser-inserted", but the element’s node document is not the Document of the parser that created the element, then abort these steps.

  10. If scripting is disabled for the script element, then abort these steps at this point. The script is not executed.

    The definition of scripting is disabled means that, amongst others, the following scripts will not execute: scripts in XMLHttpRequest's responseXML documents, scripts in DOMParser-created documents, scripts in documents created by XSLTProcessor’s transformToDocument feature, and scripts that are first inserted by a script into a Document that was created using the createDocument() API. [XHR] [DOM-Parsing] [DOM]

  11. If the script element does not have a src content attribute, and the Should element’s inline behavior be blocked by Content Security Policy? algorithm returns "Blocked" when executed upon the script element, "script", and the script element’s child text content, then abort these steps. The script is not executed. [CSP3]

  12. If the script element has an event attribute and a for attribute, and the script’s type is "classic", then run these substeps:

    1. Let for be the value of the for attribute.

    2. Let event be the value of the event attribute.

    3. Strip leading and trailing whitespace from event and for.

    4. If for is not an ASCII case-insensitive match for the string "window", then the user agent must abort these steps at this point. The script is not executed.

    5. If event is not an ASCII case-insensitive match for either the string "onload" or the string "`onload()`", then the user agent must abort these steps at this point. The script is not executed.

  13. If the script element has a charset attribute, then let encoding be the result of getting an encoding from the value of the charset attribute.

    If the script element does not have a charset attribute, or if getting an encoding failed, let encoding be the same as the encoding of the document itself.

  14. Let CORS setting be the current state of the element’s crossorigin content attribute.

  15. If the script element has a nonce attribute, then let cryptographic nonce be that attribute’s value.

    Otherwise, let cryptographic nonce be the empty string.

  16. Let parser state be "parser-inserted" if the script element has been flagged as "parser-inserted", and "`not parser-inserted`" otherwise.

  17. Let settings be the element’s node document’s Window object’s environment settings object.

  18. If the element has a src content attribute, run these substeps:

    1. Let src be the value of the element’s src attribute.

    2. If src is the empty string, queue a task to fire a simple event named error at the element, and abort these steps.

    3. Set the element’s from an external file flag.

    4. Parse src relative to the element’s node document.

    5. If the previous step failed, queue a task to fire a simple event named error at the element, and abort these steps.

      Otherwise, let url be the resulting URL record.

    6. Switch on the script’s type:

      `"classic"`
      Fetch a classic script given url, CORS setting, cryptographic nonce, parser state, settings, and encoding.
      When the chosen algorithm asynchronously completes, set the script’s script to the result. At that time, the script is ready.

      For performance reasons, user agents may start fetching the classic script (as defined above) as the src attribute is set, instead, in the hope that the element will be inserted into the document (and that the crossorigin attribute won’t change value in the meantime). Either way, once the element is inserted into the document, the load must have started as described in this step. If the UA performs such prefetching, but the element is never inserted in the document, or the src attribute is dynamically changed, or the crossorigin attribute is dynamically changed, then the user agent will not execute the script so obtained, and the fetching process will have been effectively wasted.

  19. If the element does not have a src content attribute, run these substeps:

    1. Let source text be the value of the text IDL attribute.

    2. Switch on the script’s type:

      `"classic"`
      1. Let script be the result of creating a classic script using source text and settings.

      2. Set the script’s script to script.

      3. The script is ready.

  20. Then, follow the first of the following options that describes the situation:

    the script’s type src present? defer present? async present? other conditions
    `"classic"` yes yes no element flagged as "parser-inserted"
    Add the element to the end of the list of scripts that will execute when the document has finished parsing associated with the Document of the parser that created the element.

    When the the script is ready, set the element’s "ready to be parser-executed" flag. The parser will handle executing the script.

    the script’s type src present? defer present? async present? other conditions
    `"classic"` yes no no element flagged as "parser-inserted"
    The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

    When the script is ready, set the element’s "ready to be parser-executed" flag. The parser will handle executing the script.

    the script’s type src present? defer present? async present? other conditions
    `"classic"` yes yes or no no "non-blocking" flag not set on element
    Add the element to the end of the list of scripts that will execute in order as soon as possible associated with the node document of the script element at the time the prepare a script algorithm started.

    When the script is ready, run the following steps:

    1. If the element is not now the first element in the list of scripts that will execute in order as soon as possible to which it was added above, then mark the element as ready but abort these steps without executing the script yet.

    2. Execution: Execute the script block corresponding to the first script element in this list of scripts that will execute in order as soon as possible.

    3. Remove the first element from this list of scripts that will execute in order as soon as possible.

    4. If this list of scripts that will execute in order as soon as possible is still not empty and the first entry has already been marked as ready, then jump back to the step labeled Execution.

    the script’s type src present? defer present? async present? other conditions
    `"classic"` yes yes or no yes or no n/a
    The element must be added to the set of scripts that will execute as soon as possible of the node document of the script element at the time the prepare a script algorithm started.

    When the script is ready, execute the script block and then remove the element from the set of scripts that will execute as soon as possible.

    the script’s type src present? defer present? async present? other conditions
    `"classic"` no yes or no yes or no All of the following:
    The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

    Set the element’s "ready to be parser-executed" flag. The parser will handle executing the script.

    Otherwise
    Immediately execute the script block, even if other scripts are already executing.

The pending parsing-blocking script of a Document is used by the Document's parser(s).

If a script element that blocks a parser gets moved to another Document before it would normally have stopped blocking that parser, it nonetheless continues blocking that parser until the condition that causes it to be blocking the parser no longer applies (e.g., if the script is a pending parsing-blocking script because there was a style sheet that is blocking scripts when it was parsed, but then the script is moved to another Document before the style sheet loads, the script still blocks the parser until the style sheets are all loaded, at which time the script executes and the parser is unblocked).

When the user agent is required to execute a script block, it must run the following steps:

  1. If the element is flagged as "parser-inserted", but the element’s node document is not the Document of the parser that created the element, then abort these steps.

  2. If the script’s script is null, fire a simple event named error at the element, and abort these steps.

  3. If the script is from an external file, then increment the ignore-destructive-writes counter of the script element’s node document. Let neutralized doc be that Document.

  4. Let old script element be the value to which the script element’s node document’s currentScript object was most recently set.

  5. Switch on the script’s type:

    `"classic"`
    1. Set the script element’s node document’s currentScript attribute to the script element.

      This does not use the in a document check, as the script element could have been removed from the document prior to execution, and in that scenario currentScript still needs to point to it.

    2. Run the classic script given by the script’s script.

  6. Set the script element’s node document’s currentScript object to old script element.

  7. Decrement the ignore-destructive-writes counter of neutralized doc, if it was incremented in the earlier step.

  8. If the script’s type is "classic" and the script is from an external file, fire a simple event named load at the script element.

    Otherwise queue a task to fire a simple event named load at the script element.

4.12.1.2. Scripting languages

A JavaScript MIME type is a MIME type string that is one of the following and refers to JavaScript: [ECMA-262]

User agents must recognize all JavaScript MIME types.

User agents may support other MIME types for other languages, but must not support other MIME types for the languages in the list above. User agents are not required to support JavaScript. The processing model for languages other than JavaScript is outside the scope of this specification.

The following MIME types (with or without parameters) must not be interpreted as scripting languages:

These types are explicitly listed here because they are poorly-defined types that are nonetheless likely to be used as formats for data blocks, and it would be problematic if they were suddenly to be interpreted as script by a user agent.

When examining types to determine if they represent supported languages, user agents must not ignore MIME parameters. Types are to be compared including all parameters.

For example, types that include the charset parameter will not be recognized as referencing any of the scripting languages listed above.

4.12.1.3. 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 "`<!--`" as "`<\!--`", "`<script`" as "`<\script`", and "`</script`" as "`<\/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 textContent of a script element 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.

The following script illustrates this issue. Suppose you have a script that contains a string, as in:
var example = 'Consider this string: <!-- <script>';
console.log(example);

If one were to put this string directly in a script block, it would violate the restrictions above:

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

The bigger problem, though, and the reason why it would violate those restrictions, is that actually the script would get parsed weirdly: the script block above is not terminated. That is, what looks like a "`</script>`" end tag in this snippet is actually still part of the script block. The script doesn’t execute (since it’s not terminated); if it somehow were to execute, as it might if the markup looked as follows, it would fail because the script is not valid JavaScript:

<script>
  var 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>

What is going on here is that for legacy reasons, "`<!--`" and "`<script`" strings in script elements in HTML need to be balanced in order for the parser to consider closing the block.

By escaping the problematic strings as mentioned at the top of this section, the problem is avoided entirely:

<script>
  var example = 'Consider this string: <\!-- <\script>';
  console.log(example);
</script>
<!-- this is just a comment between script blocks -->
<script>
  ... // this is a new script block
</script>

It is possible for these sequences to naturally occur in script expressions, as in the following examples:

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

In such cases the characters cannot be escaped, but the expressions can be rewritten so that the sequences don’t occur, as in:

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

Doing this also avoids a different pitfall as well: for related historical reasons, the string "`<!--`" in classic scripts is actually treated as a line comment start, just like "`//`".

4.12.1.4. 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 Unicode character other than U+000A LINE FEED (LF)
not-star      = %x0000-0029 / %x002B-10FFFF
                ; a Unicode character other than U+002A ASTERISK (*)
not-slash     = %x0000-002E / %x0030-10FFFF
                ; a Unicode character 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.

This allows authors to include documentation, such as license information or API information, inside their documents while still referring to external script files. The syntax is constrained so that authors don’t accidentally include what looks like valid script while also providing a src attribute.
<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>
4.12.1.5. Interaction of script elements and XSLT

This section is non-normative.

This specification does not define how XSLT interacts with the script element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, based on existing implementations:

The main distinction between the first two cases and the last case is that the first two operate on Documents and the last operates on a fragment.

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
Allowed ARIA role attribute values:
None
Allowed ARIA state and property attributes:
Global aria-* attributes
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 concatenation of all the Text node children 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.) [DOM-Parsing]

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 XHTML 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.

The noscript element has no other requirements. In particular, children of the noscript element are not exempt from §4.10.22 Form submission, scripting, and so forth, even when scripting is enabled for the element.

In the following example, a noscript element is used to provide fallback for a script.
<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>

When script is disabled, a button appears to do the calculation on the server side. When script is enabled, the value is computed on-the-fly instead.

The noscript element is a blunt instrument. Sometimes, scripts might be enabled, but for some reason the page’s script might fail. For this reason, it’s generally better to avoid using noscript, and to instead design the script to change the page from being a scriptless page to a scripted page on the fly, as in the next example:

<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>

The above technique is also useful in XHTML, since noscript is not supported in the XHTML syntax.

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:
Either: Metadata content.
Or: Flow content.
Or: The content model of ol and ul elements.
Or: The content model of dl elements.
Or: The content model of figure elements.
Or: The content model of ruby elements.
Or: The content model of object elements.
Or: The content model of video and audio elements.
Or: The content model of table elements.
Or: The content model of colgroup elements.
Or: The content model of thead, tbody, and tfoot elements.
Or: The content model of tr elements.
Or: The content model of fieldset elements.
Or: The content model of select elements.
Or: The content model of details elements.
Or: The content model of menu elements whose type attribute is in the popup menu state.
Tag omission in text/html:
Neither tag is omissible
Content attributes:
Global attributes
Allowed ARIA role attribute values:
None
Allowed ARIA state and property attributes:
Global aria-* attributes
DOM interface:
interface HTMLTemplateElement : HTMLElement {
  readonly attribute DocumentFragment content;
};

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

Templates provide a method for declaring inert DOM subtrees and manipulating them to instantiate document fragments with identical contents.

When web pages dynamically alter the contents of their documents (e.g., in response to user interaction or new data arriving from the server), it is common that they require fragments of HTML which may require further modification before use, such as the insertion of values appropriate for the usage context.

The template element allows for the declaration of document fragments which are unused by the document when loaded, but are parsed as HTML and are available at runtime for use by the web page.

In a rendering, the template element represents nothing.

template . content

Returns the contents of the template, which are stored in a DocumentFragment associated with a different Document so as to avoid the template contents interfering with the main Document. (For example, this avoids form controls from being submitted, scripts from executing, and so forth.)

Each template element has an associated DocumentFragment object that is its template contents. When a template element is created, the user agent must run the following steps to establish the template contents:

  1. Let doc be the template element’s node document’s appropriate template contents owner document.
  2. Create a DocumentFragment object whose node document is doc.
  3. Set the template element’s template contents to the newly created DocumentFragment object.

A Document doc’s appropriate template contents owner document is the Document returned by the following algorithm:

  1. If doc is not a Document created by this algorithm, run these substeps:

    1. If doc does not yet have an associated inert template document then run these substeps:

      1. Let new doc be a new Document (that does not have a browsing context). This is "a Document created by this algorithm" for the purposes of the step above.
      2. If doc is an HTML document, mark new doc as an HTML document also.
      3. Let doc’s associated inert template document be new doc.
    2. Set doc to doc’s associated inert template document.

    Each Document not created by this algorithm thus gets a single Document to act as its proxy for owning the template contents of all its template elements, so that they aren’t in a browsing context and thus remain inert (e.g., scripts do not run). Meanwhile, template elements inside Document objects that are created by this algorithm just reuse the same Document owner for their contents.

  2. Return doc.

The adopting steps (with node and oldDocument as parameters) for template elements are the following:

  1. Let doc be node’s node document’s appropriate template contents owner document.

    node’s node document is the Document object that node was just adopted into.

  2. Adopt node’s template contents (a DocumentFragment object) into doc.

The content IDL attribute must return the template element’s template contents.


The cloning steps for a template element node being cloned to a copy copy must run the following steps:

  1. If the clone children flag is not set in the calling clone algorithm, abort these steps.
  2. Let copied contents be the result of cloning all the children of node’s template contents, with document set to copy’s template contents’s node document, and with the clone children flag set.
  3. Append copied contents to copy’s template contents.
In this example, a script populates a table four-column with data from a data structure, using a template to provide the element structure instead of manually generating the structure from markup.
<!DOCTYPE html>
<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>

This example uses cloneNode() on the template’s contents; it could equivalently have used document.importNode(), which does the same thing. The only difference between these two APIs is when the node document is updated: with cloneNode() it is updated when the nodes are appended with appendChild(), with document.importNode() it is updated when the nodes are cloned.

4.12.3.1. Interaction of template elements with XSLT and XPath

This section is non-normative.

This specification does not define how XSLT and XPath interact with the template element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, which are intended to be consistent with other processing described in this specification:

  • An XSLT processor based on an XML parser that acts as described in this specification needs to act as if template elements contain as descendants their template contents for the purposes of the transform.
  • An XSLT processor that outputs a DOM needs to ensure that nodes that would go into a template element are instead placed into the element’s template contents.
  • XPath evaluation using the XPath DOM API when applied to a Document parsed using the HTML parser or the XML parser described in this specification needs to ignore template contents.

4.12.4. The canvas element

Categories:
Flow content.
Phrasing content.
Embedded content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Transparent.
Tag omission in text/html:
Neither tag is omissible
Content attributes:
Global attributes
width - Horizontal dimension
height - Vertical dimension
Allowed ARIA role attribute values:
Any role value.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;

interface HTMLCanvasElement : HTMLElement {
  attribute unsigned long width;
  attribute unsigned long height;

  RenderingContext? getContext(DOMString contextId, any... arguments);
  boolean probablySupportsContext(DOMString contextId, any... arguments);

  DOMString toDataURL(optional DOMString type, any... arguments);
  void toBlob(BlobCallback _callback, optional DOMString type, any... arguments);
};

callback BlobCallback = void (Blob? blob);

The canvas element provides scripts with a resolution-dependent bitmap canvas, which can be used for rendering graphs, game graphics, art, or other visual images on the fly.

Authors should not use the canvas element in a document when a more suitable element is available. For example, it is inappropriate to use a canvas element to render a page heading: if the desired presentation of the heading is graphically intense, it should be marked up using appropriate elements (typically h1) and then styled using CSS and supporting technologies such as Web Components.

When authors use the canvas element, they must also provide content that, when presented to the user, conveys essentially the same function or purpose as the canvas's bitmap. This content may be placed as content of the canvas element. The contents of the canvas element, if any, are the element’s fallback content.


In interactive visual media, if scripting is enabled for the canvas element, and if support for canvas elements has been enabled, the canvas element represents embedded content consisting of a dynamically created image, the element’s bitmap.

In non-interactive, static, visual media, if the canvas element has been previously associated with a rendering context (e.g., if the page was viewed in an interactive visual medium and is now being printed, or if some script that ran during the page layout process painted on the element), then the canvas element represents embedded content with the element’s current bitmap and size. Otherwise, the element represents its fallback content instead.

In non-visual media, and in visual media if scripting is disabled for the canvas element or if support for canvas elements has been disabled, the canvas element represents its fallback content instead.

When a canvas element represents embedded content, the user can still focus descendants of the canvas element (in the fallback content). When an element is focused, it is the target of keyboard interaction events (even though the element itself is not visible). This allows authors to make an interactive canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no effect on mouse interaction events.) [UIEVENTS]

An element whose nearest canvas element ancestor is being rendered and represents embedded content is an element that is being used as relevant canvas fallback content.


The canvas element has two attributes to control the size of the element’s bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.

The intrinsic dimensions of the canvas element when it represents embedded content are equal to the dimensions of the element’s bitmap.

The user agent must use a square pixel density consisting of one pixel of image data per coordinate space unit for the bitmaps of a canvas and its rendering contexts.

A canvas element can be sized arbitrarily by a style sheet, its bitmap is then subject to the object-fit CSS property. [CSS3-IMAGES]


The bitmaps of canvas elements, the bitmaps of ImageBitmap objects, as well as some of the bitmaps of rendering contexts, such as those described in the section on the CanvasRenderingContext2D object below, have an origin-clean flag, which can be set to true or false. Initially, when the canvas element or ImageBitmap object is created, its bitmap’s origin-clean flag must be set to true.

A canvas bitmap can also have a hit region list, as described in the CanvasRenderingContext2D section below.

A canvas element can have a rendering context bound to it. Initially, it does not have a bound rendering context. To keep track of whether it has a rendering context or not, and what kind of rendering context it is, a canvas also has a canvas context mode, which is initially none but can be changed to either 2d, webgl by algorithms defined in this specification.

When its canvas context mode is none, a canvas element has no rendering context, and its bitmap must be fully transparent black with an intrinsic width equal to the numeric value of the element’s width attribute and an intrinsic height equal to the numeric value of the element’s height attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or removed.

When a canvas element represents embedded content, it provides a paint source whose width is the element’s intrinsic width, whose height is the element’s intrinsic height, and whose appearance is the element’s bitmap.

Whenever the width and height content attributes are set, removed, changed, or redundantly set to the value they already have, if the canvas context mode is 2d, the user agent must set bitmap dimensions to the numeric values of the width and height content attributes.

The width and height IDL attributes must reflect the respective content attributes of the same name, with the same defaults.


context = canvas . getContext(contextId [, ... ] )

Returns an object that exposes an API for drawing on the canvas. The first argument specifies the desired API, either "2d" or "webgl". Subsequent arguments are handled by that API.

The list of defined contexts is given on the WHATWG Wiki CanvasContexts page. [WHATWGWIKI]

Example contexts are the "2d" [CANVAS-2D] and the "webgl" context [WEBGL].

Returns null if the given context ID is not supported or if the canvas has already been initialized with some other (incompatible) context type (e.g., trying to get a "2d" context after getting a "webgl" context).

supported = canvas . probablySupportsContext(contextId [, ... ] )

Returns false if calling getContext() with the same arguments would definitely return null, and true otherwise.

This return value is not a guarantee that getContext() will or will not return an object, as conditions (e.g., availability of system resources) can vary over time.

The getContext(contextId, arguments...) method of the canvas element, when invoked, must run the steps in the cell of the following table whose column header describes the canvas element’s canvas context mode and whose row header describes the method’s first argument.

getContext() invocation steps
none 2d webgl
"2d"

Set the canvas element’s context mode to 2d, obtain a CanvasRenderingContext2D object as defined in the HTML Canvas 2D Context specification [CANVAS-2D], set the obtained CanvasRenderingContext2D object’s context mode to 2d, and return the CanvasRenderingContext2D object

Return the same object as was return the last time the method was invoked with this same first argument. Return null.
"webgl", if the user agent supports the WebGL feature in its current configuration Follow the instructions given in the WebGL specification’s Context Creation section to obtain either a WebGLRenderingContext or null; if the returned value is null, then return null and abort these steps, otherwise, set the canvas element’s context mode to webgl, set the new WebGLRenderingContext object’s context mode to webgl, and return the WebGLRenderingContext object‡ [WEBGL] Return null. Return the same object as was return the last time the method was invoked with this same first argument.
A vendor-specific extension* Behave as defined for the extension. Behave as defined for the extension. Behave as defined for the extension.
An unsupported value† Return null. Return null. Return null.

* Vendors may define experimental contexts using the syntax vendorname-context, for example, moz-3d.

† For example, the "webgl" value in the case of a user agent having exhausted the graphics hardware’s abilities and having no software fallback implementation.

‡ The second (and subsequent) argument(s) to the method, if any, are ignored in all cases except this one. See the WebGL specification for details.


There is no known native implementation of the probablySupportsContext() method. Therefore this feature should not be relied upon.

The probablySupportsContext(contextId, arguments...) method of the canvas element, when invoked, must return false if calling getContext() on the same object and with the same arguments would definitely return null at this time, and true otherwise.


url = canvas . toDataURL( [ type, ... ] )

Returns a data: URL for the image in the canvas.

The first argument, if provided, controls the type of the image to be returned (e.g., PNG or JPEG). The default is image/png; that type is also used if the given type isn’t supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

When trying to use types other than "image/png", authors can check if the image was really returned in the requested format by checking to see if the returned string starts with one of the exact strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus the requested type was not supported. (The one exception to this is if the canvas has either no height or no width, in which case the result might simply be "data:,".)

canvas . toBlob(callback [, type, ... ] )

Creates a Blob object representing a file containing the image in the canvas, and invokes a callback with a handle to that object.

The second argument, if provided, controls the type of the image to be returned (e.g., PNG or JPEG). The default is image/png; that type is also used if the given type isn’t supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

The toDataURL() method must run the following steps:

  1. If the canvas element’s bitmap’s origin-clean flag is set to false, throw a "SecurityError" DOMException and abort these steps.
  2. If the canvas element’s bitmap has no pixels (i.e., either its horizontal dimension or its vertical dimension is zero) then return the string "data:," and abort these steps. (This is the shortest data: URL; it represents the empty string in a text/plain resource.)
  3. Let file be a serialization of the canvas element’s bitmap as a file, using the method’s arguments (if any) as the arguments.
  4. Return a data: URL representing file. [RFC2397]

The toBlob() method must run the following steps:

  1. If the canvas element’s bitmap’s origin-clean flag is set to false, throw a "SecurityError" DOMException and abort these steps.
  2. Let callback be the first argument.
  3. Let arguments be the second and subsequent arguments to the method, if any.
  4. If the canvas element’s bitmap has no pixels (i.e., either its horizontal dimension or its vertical dimension is zero) then let result be null.

    Otherwise, let result be a Blob object representing a serialization of the canvas element’s bitmap as a file, using arguments. [FILEAPI]

  5. Return, but continue running these steps in parallel.
  6. Queue a task to invoke the BlobCallback callback with result as its argument. The task source for this task is the canvas blob serialization task source.
4.12.4.1. Color spaces and color correction

The canvas APIs must perform color correction at only two points: when rendering images with their own gamma correction and color space information onto a bitmap, to convert the image to the color space used by the bitmaps (e.g., using the 2D Context’s drawImage() method with an HTMLImageElement object), and when rendering the actual canvas bitmap to the output device.

Thus, in the 2D context, colors used to draw shapes onto the canvas will exactly match colors obtained through the getImageData() method.

The toDataURL() method must not include color space information in the resources they return. Where the output format allows it, the color of pixels in resources created by toDataURL() must match those returned by the getImageData() method.

In user agents that support CSS, the color space used by a canvas element must match the color space used for processing any colors for that element in CSS.

The gamma correction and color space information of images must be handled in such a way that an image rendered directly using an img element would use the same colors as one painted on a canvas element that is then itself rendered. Furthermore, the rendering of images that have no color correction information (such as those returned by the toDataURL() method) must be rendered with no color correction.

Thus, in the 2D context, calling the drawImage() method to render the output of the toDataURL() method to the canvas, given the appropriate dimensions, has no visible effect.

4.12.4.2. Serializing bitmaps to a file

When a user agent is to create a serialization of the bitmap as a file, optionally with some given arguments, and optionally with a native flag set, it must create an image file in the format given by the first value of arguments, or, if there are no arguments, in the PNG format. [PNG]

If the native flag is set, or if the bitmap has one pixel per coordinate space unit, then the image file must have the same pixel data (before compression, if applicable) as the bitmap, and if the file format used supports encoding resolution metadata, the resolution of that bitmap (device pixels per coordinate space units being interpreted as image pixels per CSS pixel) must be given as well.

Otherwise, the image file’s pixel data must be the bitmap’s pixel data scaled to one image pixel per coordinate space unit, and if the file format used supports encoding resolution metadata, the resolution must be given as 96dpi (one image pixel per CSS pixel).

If arguments is not empty, the first value must be interpreted as a MIME type giving the format to use. If the type has any parameters, it must be treated as not supported.

For example, the value "image/png" would mean to generate a PNG image, the value "image/jpeg" would mean to generate a JPEG image, and the value "image/svg+xml" would mean to generate an SVG image (which would require that the user agent track how the bitmap was generated, an unlikely, though potentially awesome, feature).

User agents must support PNG ("image/png"). User agents may support other types. If the user agent does not support the requested type, it must create the file using the PNG format. [PNG]

User agents must convert the provided type to ASCII lowercase before establishing if they support that type.

For image types that do not support an alpha channel, the serialized image must be the bitmap image composited onto a solid black background using the source-over operator.

If the first argument in arguments gives a type corresponding to one of the types given in the first column of the following table, and the user agent supports that type, then the subsequent arguments, if any, must be treated as described in the second cell of that row.

Arguments for serialization methods
Type Other arguments Reference
image/jpeg The second argument, if it is a number in the range 0.0 to 1.0 inclusive, must be treated as the desired quality level. If it is not a number or is outside that range, the user agent must use its default value, as if the argument had been omitted. [JPEG]

For the purposes of these rules, an argument is considered to be a number if it is converted to an IDL double value by the rules for handling arguments of type any in the Web IDL specification. [WEBIDL]

Other arguments must be ignored and must not cause the user agent to throw an exception. A future version of this specification will probably define other parameters to be passed to these methods to allow authors to more carefully control compression settings, image metadata, etc.

4.12.4.3. Security with canvas elements

This section is non-normative.

Information leakage can occur if scripts from one origin can access information (e.g., read pixels) from images from another origin (one that isn’t the same).

To mitigate this, bitmaps used with canvas elements and ImageBitmap objects are defined to have a flag indicating whether they are origin-clean. All bitmaps start with their origin-clean set to true. The flag is set to false when cross-origin images or fonts are used.

The toDataURL(), toBlob(), and getImageData() methods check the flag and will throw a "SecurityError" DOMException rather than leak cross-origin data.

The value of the origin-clean flag is propagated from a source canvas element’s bitmap to a new ImageBitmap object by createImageBitmap(). Conversely, a destination canvas element’s bitmap will have its origin-clean flags set to false by drawImage if the source image is an ImageBitmap object whose bitmap has its origin-clean flag set to false.

The flag can be reset in certain situations; for example, when a CanvasRenderingContext2D is bound to a new canvas, the bitmap is cleared and its flag reset.