HTML

Living Standard — Last Updated 8 November 2024

1. 1. 1. 4.8.5 The iframe element
      2. 4.8.6 The embed element
      3. 4.8.7 The object element

4.8.5 The iframe element

Categories:

Flow content.

Phrasing content.

Embedded content.

Interactive content.

Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

Nothing.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

src — Address of the resource

srcdoc — A document to render in the iframe

name — Name of content navigable

sandbox — Security rules for nested content

allow — Permissions policy to be applied to the iframe's contents

allowfullscreen — Whether to allow the iframe's contents to use requestFullscreen()

width — Horizontal dimension

height — Vertical dimension

referrerpolicy — Referrer policy for fetches initiated by the element

loading — Used when determining loading deferral

Accessibility considerations:

For authors.

For implementers.

DOM interface:

[Exposed=Window]
interface HTMLIFrameElement : HTMLElement {
  [HTMLConstructor] constructor();

  [CEReactions] attribute USVString src;
  [CEReactions] attribute (TrustedHTML or DOMString) srcdoc;
  [CEReactions] attribute DOMString name;
  [SameObject, PutForwards=value] readonly attribute DOMTokenList sandbox;
  [CEReactions] attribute DOMString allow;
  [CEReactions] attribute boolean allowFullscreen;
  [CEReactions] attribute DOMString width;
  [CEReactions] attribute DOMString height;
  [CEReactions] attribute DOMString referrerPolicy;
  [CEReactions] attribute DOMString loading;
  readonly attribute Document? contentDocument;
  readonly attribute WindowProxy? contentWindow;
  Document? getSVGDocument();

  // also has obsolete members
};

The iframe element represents its content navigable.

The src attribute gives the URL of a page that the element's content navigable is to contain. The attribute, if present, must be a valid non-empty URL potentially surrounded by spaces. If the itemprop attribute is specified on an iframe element, then the src attribute must also be specified.

The srcdoc attribute gives the content of the page that the element's content navigable is to contain. The value of the attribute is used to construct an iframe srcdoc document, which is a Document whose URL matches about:srcdoc.

The srcdoc attribute, if present, must have a value using the HTML syntax that consists of the following syntactic components, in the given order:

1. Any number of comments and ASCII whitespace.
2. Optionally, a DOCTYPE.
3. Any number of comments and ASCII whitespace.
4. The document element, in the form of an html element.
5. Any number of comments and ASCII whitespace.

The above requirements apply in XML documents as well.

<article>
 <h1>I got my own magazine!</h1>
 <p>After much effort, I've finally found a publisher, and so now I
 have my own magazine! Isn't that awesome?! The first issue will come
 out in September, and we have articles about getting food, and about
 getting in boxes, it's going to be great!</p>
 <footer>
  <p>Written by <a href="/users/cap">cap</a>, 1 hour ago.
 </footer>
 <article>
  <footer> Thirteen minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe sandbox srcdoc="<p>did you get a cover picture yet?"></iframe>
 </article>
 <article>
  <footer> Nine minutes ago, <a href="/users/cap">cap</a> wrote: </footer>
  <iframe sandbox srcdoc="<p>Yeah, you can see it <a href=&quot;/gallery?mode=cover&amp;amp;page=1&quot;>in my gallery</a>."></iframe>
 </article>
 <article>
  <footer> Five minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe sandbox srcdoc="<p>hey that's earl's table.
<p>you should get earl&amp;amp;me on the next cover."></iframe>
 </article>

In the HTML syntax, authors need only remember to use U+0022 QUOTATION MARK characters (") to wrap the attribute contents and then to escape all U+0026 AMPERSAND (&) and U+0022 QUOTATION MARK (") characters, and to specify the sandbox attribute, to ensure safe embedding of content. (And remember to escape ampersands before quotation marks, to ensure quotation marks become " and not ".)

In XML the U+003C LESS-THAN SIGN character (<) needs to be escaped as well. In order to prevent attribute-value normalization, some of XML's whitespace characters — specifically U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), and U+000D CARRIAGE RETURN (CR) — also need to be escaped. [XML]

If the src attribute and the srcdoc attribute are both specified together, the srcdoc attribute takes priority. This allows authors to provide a fallback URL for legacy user agents that do not support the srcdoc attribute.

The iframe HTML element insertion steps, given insertedNode, are:

1. If insertedNode's shadow-including root's browsing context is null, then return.

2. Create a new child navigable for insertedNode.

3. If insertedNode has a sandbox attribute, then parse the sandboxing directive given the attribute's value and insertedNode's iframe sandboxing flag set.

4. Process the iframe attributes for insertedNode, with initialInsertion set to true.

The iframe HTML element removing steps, given removedNode, are to destroy a child navigable given removedNode.

This happens without any unload events firing (the element's content document is destroyed, not unloaded).

Although iframes are processed while in a shadow tree, per the above, several other aspects of their behavior are not well-defined with regards to shadow trees. See issue #763 for more detail.

Whenever an iframe element with a non-null content navigable has its srcdoc attribute set, changed, or removed, the user agent must process the iframe attributes.

Similarly, whenever an iframe element with a non-null content navigable but with no srcdoc attribute specified has its src attribute set, changed, or removed, the user agent must process the iframe attributes.

To process the iframe attributes for an element element, with an optional boolean initialInsertion (default false):

1. If element's srcdoc attribute is specified, then:

   1. Set element's current navigation was lazy loaded boolean to false.

   2. If the will lazy load element steps given element return true, then:

      1. Set element's lazy load resumption steps to the rest of this algorithm starting with the step labeled navigate to the srcdoc resource.

      2. Set element's current navigation was lazy loaded boolean to true.

      3. Start intersection-observing a lazy loading element for element.

      4. Return.

   3. Navigate to the srcdoc resource: Navigate an iframe or frame given element, about:srcdoc, the empty string, and the value of element's srcdoc attribute.

      The resulting Document must be considered an iframe srcdoc document.

2. Otherwise:

   1. Let url be the result of running the shared attribute processing steps for iframe and frame elements given element and initialInsertion.

   2. If url is null, then return.

   3. If url matches about:blank and initialInsertion is true, then:

      1. Run the iframe load event steps given element.

      2. Return.

   4. Let referrerPolicy be the current state of element's referrerpolicy content attribute.

   5. Set element's current navigation was lazy loaded boolean to false.

   6. If the will lazy load element steps given element return true, then:

      1. Set element's lazy load resumption steps to the rest of this algorithm starting with the step labeled navigate.

      2. Set element's current navigation was lazy loaded boolean to true.

      3. Start intersection-observing a lazy loading element for element.

      4. Return.

   7. Navigate: Navigate an iframe or frame given element, url, and referrerPolicy.

The shared attribute processing steps for iframe and frame elements, given an element element and a boolean initialInsertion, are:

1. Let url be the URL record about:blank.

2. If element has a src attribute specified, and its value is not the empty string, then:

   1. Let maybeURL be the result of encoding-parsing a URL given that attribute's value, relative to element's node document.

   2. If maybeURL is not failure, then set url to maybeURL.

3. If the inclusive ancestor navigables of element's node navigable contains a navigable whose active document's URL equals url with exclude fragments set to true, then return null.

4. If url matches about:blank and initialInsertion is true, then perform the URL and history update steps given element's content navigable's active document and url.

   This is necessary in case url is something like about:blank?foo. If url is just plain about:blank, this will do nothing.

5. Return url.

To navigate an iframe or frame given an element element, a URL url, a referrer policy referrerPolicy, and an optional string-or-null srcdocString (default null):

1. Let historyHandling be "auto".

2. If element's content navigable's active document is not completely loaded, then set historyHandling to "replace".

3. If element is an iframe, then set element's pending resource-timing start time to the current high resolution time given element's node document's relevant global object.

4. Navigate element's content navigable to url using element's node document, with historyHandling set to historyHandling, referrerPolicy set to referrerPolicy, and documentResource set to srcdocString.

Each Document has an iframe load in progress flag and a mute iframe load flag. When a Document is created, these flags must be unset for that Document.

To run the iframe load event steps, given an iframe element element:

1. Assert: element's content navigable is not null.

2. Let childDocument be element's content navigable's active document.

3. If childDocument has its mute iframe load flag set, then return.

4. If element's pending resource-timing start time is not null, then:

   1. Let global be element's node document's relevant global object.

   2. Let fallbackTimingInfo be a new fetch timing info whose start time is element's pending resource-timing start time and whose response end time is the current high resolution time given global.

   3. Mark resource timing given fallbackTimingInfo, url, "iframe", global, the empty string, a new response body info, and 0.

   4. Set element's pending resource-timing start time to null.

5. Set childDocument's iframe load in progress flag.

6. Fire an event named load at element.

7. Unset childDocument's iframe load in progress flag.

This, in conjunction with scripting, can be used to probe the URL space of the local network's HTTP servers. User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing web content.

If an element type potentially delays the load event, then for each element element of that type, the user agent must delay the load event of element's node document if element's content navigable is non-null and any of the following are true:

• element's content navigable's active document is not ready for post-load tasks;

• element's content navigable's is delaying load events is true; or

• anything is delaying the load event of element's content navigable's active document.

If, during the handling of the load event, element's content navigable is again navigated, that will further delay the load event.

Each iframe element has an associated current navigation was lazy loaded boolean, initially false. It is set and unset in the process the iframe attributes algorithm.

An iframe element whose current navigation was lazy loaded boolean is false potentially delays the load event.

Each iframe element has an associated null or DOMHighResTimeStamp pending resource-timing start time, initially set to null.

If, when the element is created, the srcdoc attribute is not set, and the src attribute is either also not set or set but its value cannot be parsed, the element's content navigable will remain at the initial about:blank Document.

If the user navigates away from this page, the iframe's content navigable's active WindowProxy object will proxy new Window objects for new Document objects, but the src attribute will not change.

The name attribute, if present, must be a valid navigable target name. The given value is used to name the element's content navigable if present when that is created.

The sandbox attribute, when specified, enables a set of extra restrictions on any content hosted by the iframe. Its value must be an unordered set of unique space-separated tokens that are ASCII case-insensitive. The allowed values are:

• allow-downloads
• allow-forms
• allow-modals
• allow-orientation-lock
• allow-pointer-lock
• allow-popups
• allow-popups-to-escape-sandbox
• allow-presentation
• allow-same-origin
• allow-scripts
• allow-top-navigation
• allow-top-navigation-by-user-activation
• allow-top-navigation-to-custom-protocols

When the attribute is set, the content is treated as being from a unique opaque origin, forms, scripts, and various potentially annoying APIs are disabled, and links are prevented from targeting other navigables. The allow-same-origin keyword causes the content to be treated as being from its real origin instead of forcing it into an opaque origin; the allow-top-navigation keyword allows the content to navigate its traversable navigable; the allow-top-navigation-by-user-activation keyword behaves similarly but allows such navigation only when the browsing context's active window has transient activation; the allow-top-navigation-to-custom-protocols reenables navigations toward non fetch scheme to be handed off to external software; and the allow-forms, allow-modals, allow-orientation-lock, allow-pointer-lock, allow-popups, allow-presentation, allow-scripts, and allow-popups-to-escape-sandbox keywords re-enable forms, modal dialogs, screen orientation lock, the pointer lock API, popups, the presentation API, scripts, and the creation of unsandboxed auxiliary browsing contexts respectively. The allow-downloads keyword allows content to perform downloads. [POINTERLOCK] [SCREENORIENTATION] [PRESENTATION]

The allow-top-navigation and allow-top-navigation-by-user-activation keywords must not both be specified, as doing so is redundant; only allow-top-navigation will have an effect in such non-conformant markup.

Similarly, the allow-top-navigation-to-custom-protocols keyword must not be specified if either allow-top-navigation or allow-popups are specified, as doing so is redundant.

To allow alert(), confirm(), and prompt() inside sandboxed content, both the allow-modals and allow-same-origin keywords need to be specified, and the loaded URL needs to be same origin with the top-level origin. Without the allow-same-origin keyword, the content is always treated as cross-origin, and cross-origin content cannot show simple dialogs.

Setting both the allow-scripts and allow-same-origin keywords together when the embedded page has the same origin as the page containing the iframe allows the embedded page to simply remove the sandbox attribute and then reload itself, effectively breaking out of the sandbox altogether.

These flags only take effect when the content navigable of the iframe element is navigated. Removing them, or removing the entire sandbox attribute, has no effect on an already-loaded page.

Potentially hostile files should not be served from the same server as the file containing the iframe element. Sandboxing hostile content is of minimal help if an attacker can convince the user to just visit the hostile content directly, rather than in the iframe. To limit the damage that can be caused by hostile HTML content, it should be served from a separate dedicated domain. Using a different domain ensures that scripts in the files are unable to attack the site, even if the user is tricked into visiting those pages directly, without the protection of the sandbox attribute.

When an iframe element's sandbox attribute is set or changed while it has a non-null content navigable, the user agent must parse the sandboxing directive given the attribute's value and the iframe element's iframe sandboxing flag set.

When an iframe element's sandbox attribute is removed while it has a non-null content navigable, the user agent must empty the iframe element's iframe sandboxing flag set.

<p>We're not scared of you! Here is your content, unedited:</p>
<iframe sandbox src="https://usercontent.example.net/getusercontent.cgi?id=12193"></iframe>

<iframe sandbox="allow-same-origin allow-forms allow-scripts"
        src="https://maps.example.com/embedded.html"></iframe>

<iframe sandbox="allow-same-origin allow-forms" src=B></iframe>

<iframe sandbox="allow-scripts" src=C></iframe>

<a href=D>Link</a>

The allow attribute, when specified, determines the container policy that will be used when the permissions policy for a Document in the iframe's content navigable is initialized. Its value must be a serialized permissions policy. [PERMISSIONSPOLICY]

<iframe src="https://maps.example.com/" allow="geolocation"></iframe>

The allowfullscreen attribute is a boolean attribute. When specified, it indicates that Document objects in the iframe element's content navigable will be initialized with a permissions policy which allows the "fullscreen" feature to be used from any origin. This is enforced by the process permissions policy attributes algorithm. [PERMISSIONSPOLICY]

<article>
 <header>
  <p><img src="/usericons/1627591962735"> <b>Fred Flintstone</b></p>
  <p><a href="/posts/3095182851" rel=bookmark>12:44</a> — <a href="#acl-3095182851">Private Post</a></p>
 </header>
 <p>Check out my new ride!</p>
 <iframe src="https://video.example.com/embed?id=92469812" allowfullscreen></iframe>
</article>

Neither allow nor allowfullscreen can grant access to a feature in an iframe element's content navigable if the element's node document is not already allowed to use that feature.

To determine whether a Document object document is allowed to use the policy-controlled-feature feature, run these steps:

1. If document's browsing context is null, then return false.

2. If document is not fully active, then return false.

3. If the result of running is feature enabled in document for origin on feature, document, and document's origin is "Enabled", then return true.

4. Return false.

Because they only influence the permissions policy of the content navigable's active document, the allow and allowfullscreen attributes only take effect when the content navigable of the iframe is navigated. Adding or removing them has no effect on an already-loaded document.

The iframe element supports dimension attributes for cases where the embedded content has specific dimensions (e.g. ad units have well-defined dimensions).

An iframe element never has fallback content, as it will always create a new child navigable, regardless of whether the specified initial contents are successfully used.

The referrerpolicy attribute is a referrer policy attribute. Its purpose is to set the referrer policy used when processing the iframe attributes. [REFERRERPOLICY]

The loading attribute is a lazy loading attribute. Its purpose is to indicate the policy for loading iframe elements that are outside the viewport.

When the loading attribute's state is changed to the Eager state, the user agent must run these steps:

1. Let resumptionSteps be the iframe element's lazy load resumption steps.

2. If resumptionSteps is null, then return.

3. Set the iframe's lazy load resumption steps to null.

4. Invoke resumptionSteps.

Descendants of iframe elements represent nothing. (In legacy user agents that do not support iframe elements, the contents would be parsed as markup that could act as fallback content.)

The HTML parser treats markup inside iframe elements as text.

The IDL attributes src, name, sandbox, and allow must reflect the respective content attributes of the same name.

The srcdoc getter steps are:

1. Let attribute be the result of running get an attribute by namespace and local name given null, srcdoc's local name, and this.

2. If attribute is null, then return the empty string.

3. Return attribute's value.

The srcdoc setter steps are:

1. Let compliantString be the result of invoking the Get Trusted Type compliant string algorithm with TrustedHTML, this's relevant global object, the given value, "HTMLIFrameElement srcdoc", and "script".

2. Set an attribute value given this, srcdoc's local name, and compliantString.

The supported tokens for sandbox's DOMTokenList are the allowed values defined in the sandbox attribute and supported by the user agent.

The allowFullscreen IDL attribute must reflect the allowfullscreen content attribute.

The referrerPolicy IDL attribute must reflect the referrerpolicy content attribute, limited to only known values.

The loading IDL attribute must reflect the loading content attribute, limited to only known values.

The contentDocument getter steps are to return the this's content document.

The contentWindow getter steps are to return this's content window.

<iframe src="https://ads.example.com/?customerid=923513721&amp;format=banner"
        width="468" height="60"></iframe>

4.8.6 The embed element

Categories:

Flow content.

Phrasing content.

Embedded content.

Interactive content.

Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

Nothing.

Tag omission in text/html:

No end tag.

Content attributes:

Global attributes

src — Address of the resource

type — Type of embedded resource

width — Horizontal dimension

height — Vertical dimension

Any other attribute that has no namespace (see prose).

Accessibility considerations:

For authors.

For implementers.

DOM interface:

[Exposed=Window]
interface HTMLEmbedElement : HTMLElement {
  [HTMLConstructor] constructor();

  [CEReactions] attribute USVString src;
  [CEReactions] attribute DOMString type;
  [CEReactions] attribute DOMString width;
  [CEReactions] attribute DOMString height;
  Document? getSVGDocument();

  // also has obsolete members
};

The embed element provides an integration point for an external application or interactive content.

The src attribute gives the URL of the resource being embedded. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

If the itemprop attribute is specified on an embed element, then the src attribute must also be specified.

The type attribute, if present, gives the MIME type by which the plugin to instantiate is selected. The value must be a valid MIME type string. If both the type attribute and the src attribute are present, then the type attribute must specify the same type as the explicit Content-Type metadata of the resource given by the src attribute.

While any of the following conditions are occurring, any plugin instantiated for the element must be removed, and the embed element represents nothing:

• The element has neither a src attribute nor a type attribute.

• The element has a media element ancestor.

• The element has an ancestor object element that is not showing its fallback content.

An embed element is said to be potentially active when the following conditions are all met simultaneously:

• The element is in a document or was in a document the last time the event loop reached step 1.

• The element's node document is fully active.

• The element has either a src attribute set or a type attribute set (or both).

• The element's src attribute is either absent or its value is not the empty string.

• The element is not a descendant of a media element.

• The element is not a descendant of an object element that is not showing its fallback content.

• The element is being rendered, or was being rendered the last time the event loop reached step 1.

Whenever an embed element that was not potentially active becomes potentially active, and whenever a potentially active embed element that is remaining potentially active and has its src attribute set, changed, or removed or its type attribute set, changed, or removed, the user agent must queue an element task on the embed task source given the element to run the embed element setup steps for that element.

The embed element setup steps for a given embed element element are as follows:

1. If another task has since been queued to run the embed element setup steps for element, then return.

2. If element has a src attribute set, then:

   1. Let url be the result of encoding-parsing a URL given element's src attribute's value, relative to element's node document.

   2. If url is failure, then return.

   3. Let request be a new request whose URL is url, client is element's node document's relevant settings object, destination is "embed", credentials mode is "include", mode is "navigate", initiator type is "embed", and whose use-URL-credentials flag is set.

   4. Fetch request, with processResponse set to the following steps given response response:

      1. If another task has since been queued to run the embed element setup steps for element, then return.

      2. If response is a network error, then fire an event named load at element, and return.

      3. Let type be the result of determining the type of content given element and response.

      4. Switch on type:

         null

         1. Display no plugin for element.

         Otherwise

         1. If element's content navigable is null, then create a new child navigable for element.

         2. Navigate element's content navigable to response's URL using element's node document, with response set to response, and historyHandling set to "replace".

            element's src attribute does not get updated if the content navigable gets further navigated to other locations.

         3. element now represents its content navigable.

      Fetching the resource must delay the load event of element's node document.

3. Otherwise, display no plugin for element.

To determine the type of the content given an embed element element and a response response, run the following steps:

1. If element has a type attribute, and that attribute's value is a type that a plugin supports, then return the value of the type attribute.

2. If the path component of response's url matches a pattern that a plugin supports, then return the type that that plugin can handle.

   For example, a plugin might say that it can handle URLs with path components that end with the four character string ".swf".

3. If response has explicit Content-Type metadata, and that value is a type that a plugin supports, then return that value.

4. Return null.

It is intentional that the above algorithm allows response to have a non-ok status. This allows servers to return data for plugins even with error responses (e.g., HTTP 500 Internal Server Error codes can still contain plugin data).

To display no plugin for an embed element element:

1. Destroy a child navigable given element.

2. Display an indication that no plugin could be found for element, as the contents of element.

3. element now represents nothing.

The embed element has no fallback content; its descendants are ignored.

Whenever an embed element that was potentially active stops being potentially active, any plugin that had been instantiated for that element must be unloaded.

The embed element potentially delays the load event.

The embed element supports dimension attributes.

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

4.8.7 The object element

Categories:

Flow content.

Phrasing content.

Embedded content.

Listed form-associated element.

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

data — Address of the resource

type — Type of embedded resource

name — Name of content navigable

form — Associates the element with a form element

width — Horizontal dimension

height — Vertical dimension

Accessibility considerations:

For authors.

For implementers.

DOM interface:

[Exposed=Window]
interface HTMLObjectElement : HTMLElement {
  [HTMLConstructor] constructor();

  [CEReactions] attribute USVString data;
  [CEReactions] attribute DOMString type;
  [CEReactions] attribute DOMString name;
  readonly attribute HTMLFormElement? form;
  [CEReactions] attribute DOMString width;
  [CEReactions] attribute DOMString height;
  readonly attribute Document? contentDocument;
  readonly attribute WindowProxy? contentWindow;
  Document? getSVGDocument();

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  boolean reportValidity();
  undefined setCustomValidity(DOMString error);

  // also has obsolete members
};

Depending on the type of content instantiated by the object element, the node also supports other interfaces.

The object element can represent an external resource, which, depending on the type of the resource, will either be treated as an image or as a child navigable.

The data attribute specifies the URL of the resource. It must be present, and must contain a valid non-empty URL potentially surrounded by spaces.

The type attribute, if present, specifies the type of the resource. If present, the attribute must be a valid MIME type string.

The name attribute, if present, must be a valid navigable target name. The given value is used to name the element's content navigable, if applicable, and if present when the element's content navigable is created.

Whenever one of the following conditions occur:

• the element is created,
• the element is popped off the stack of open elements of an HTML parser or XML parser,
• the element is not on the stack of open elements of an HTML parser or XML parser, and it is either inserted into a document or removed from a document,
• the element's node document changes whether it is fully active,
• one of the element's ancestor object elements changes to or from showing its fallback content,
• the element's classid attribute is set, changed, or removed,
• the element's classid attribute is not present, and its data attribute is set, changed, or removed,
• neither the element's classid attribute nor its data attribute are present, and its type attribute is set, changed, or removed,
• the element changes from being rendered to not being rendered, or vice versa,

...the user agent must queue an element task on the DOM manipulation task source given the object element to run the following steps to (re)determine what the object element represents. This task being queued or actively running must delay the load event of the element's node document.

1. If the user has indicated a preference that this object element's fallback content be shown instead of the element's usual behavior, then jump to the step below labeled fallback.

   For example, a user could ask for the element's fallback content to be shown because that content uses a format that the user finds more accessible.

2. If the element has an ancestor media element, or has an ancestor object element that is not showing its fallback content, or if the element is not in a document whose browsing context is non-null, or if the element's node document is not fully active, or if the element is still in the stack of open elements of an HTML parser or XML parser, or if the element is not being rendered, then jump to the step below labeled fallback.

3. If the data attribute is present and its value is not the empty string, then:

   1. If the type attribute is present and its value is not a type that the user agent supports, then the user agent may jump to the step below labeled fallback without fetching the content to examine its real type.

   2. Let url be the result of encoding-parsing a URL given the data attribute's value, relative to the element's node document.

   3. If url is failure, then fire an event named error at the element and jump to the step below labeled fallback.

   4. Let request be a new request whose URL is url, client is the element's node document's relevant settings object, destination is "object", credentials mode is "include", mode is "navigate", initiator type is "object", and whose use-URL-credentials flag is set.

   5. Fetch request.

      Fetching the resource must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined next) has been run.

   6. If the resource is not yet available (e.g. because the resource was not available in the cache, so that loading the resource required making a request over the network), then jump to the step below labeled fallback. The task that is queued by the networking task source once the resource is available must restart this algorithm from this step. Resources can load incrementally; user agents may opt to consider a resource "available" whenever enough data has been obtained to begin processing the resource.

   7. If the load failed (e.g. there was an HTTP 404 error, there was a DNS error), fire an event named error at the element, then jump to the step below labeled fallback.

   8. Determine the resource type, as follows:

      1. Let the resource type be unknown.

      2. If the user agent is configured to strictly obey Content-Type headers for this resource, and the resource has associated Content-Type metadata, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.

         This can introduce a vulnerability, wherein a site is trying to embed a resource that uses a particular type, but the remote site overrides that and instead furnishes the user agent with a resource that triggers a different type of content with different security characteristics.

      3. Run the appropriate set of steps from the following list:

         If the resource has associated Content-Type metadata

         1. Let binary be false.

         2. If the type specified in the resource's Content-Type metadata is "text/plain", and the result of applying the rules for distinguishing if a resource is text or binary to the resource is that the resource is not text/plain, then set binary to true.

         3. If the type specified in the resource's Content-Type metadata is "application/octet-stream", then set binary to true.

         4. If binary is false, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.

         5. If there is a type attribute present on the object element, and its value is not application/octet-stream, then run the following steps:

            1. If the attribute's value is a type that starts with "image/" that is not also an XML MIME type, then let the resource type be the type specified in that type attribute.

            2. Jump to the step below labeled handler.

         Otherwise, if the resource does not have associated Content-Type metadata

         1. If there is a type attribute present on the object element, then let the tentative type be the type specified in that type attribute.

            Otherwise, let tentative type be the computed type of the resource.

         2. If tentative type is not application/octet-stream, then let resource type be tentative type and jump to the step below labeled handler.

      4. If applying the URL parser algorithm to the URL of the specified resource (after any redirects) results in a URL record whose path component matches a pattern that a plugin supports, then let resource type be the type that that plugin can handle.

         For example, a plugin might say that it can handle resources with path components that end with the four character string ".swf".

      It is possible for this step to finish, or for one of the substeps above to jump straight to the next step, with resource type still being unknown. In both cases, the next step will trigger fallback.

   9. Handler: Handle the content as given by the first of the following cases that matches:

      If the resource type is an XML MIME type, or if the resource type does not start with "image/"

      If the object element's content navigable is null, then create a new child navigable for the element.

      Let response be the response from fetch.

      If response's URL does not match about:blank, then navigate the element's content navigable to response's URL using the element's node document, with historyHandling set to "replace".

      The data attribute of the object element doesn't get updated if the content navigable gets further navigated to other locations.

      The object element represents its content navigable.

      If the resource type starts with "image/", and support for images has not been disabled

      Destroy a child navigable given the object element.

      Apply the image sniffing rules to determine the type of the image.

      The object element represents the specified image.

      If the image cannot be rendered, e.g. because it is malformed or in an unsupported format, jump to the step below labeled fallback.

      Otherwise

      The given resource type is not supported. Jump to the step below labeled fallback.

      If the previous step ended with the resource type being unknown, this is the case that is triggered.

   10. The element's contents are not part of what the object element represents.

   11. If the object element does not represent its content navigable, then once the resource is completely loaded, queue an element task on the DOM manipulation task source given the object element to fire an event named load at the element.

       If the element does represent its content navigable, then an analogous task will be queued when the created Document is completely finished loading.

   12. Return.

4. Fallback: The object element represents the element's children. This is the element's fallback content. Destroy a child navigable given the element.

Due to the algorithm above, the contents of object elements act as fallback content, used only when referenced resources can't be shown (e.g. because it returned a 404 error). This allows multiple object elements to be nested inside each other, targeting multiple user agents with different capabilities, with the user agent picking the first one it supports.

The object element potentially delays the load event.

The form attribute is used to explicitly associate the object element with its form owner.

The object element supports dimension attributes.

The IDL attributes data, type, and name each must reflect the respective content attributes of the same name.

The contentDocument getter steps are to return this's content document.

The contentWindow getter steps are to return this's content window.

The willValidate, validity, and validationMessage attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the constraint validation API. The form IDL attribute is part of the element's forms API.

<figure>
 <object data="clock.html"></object>
 <figcaption>My HTML Clock</figcaption>
</figure>