HTML

Living Standard — Last Updated 28 September 2020

1. 1. 7.10 Browsing the web
      1. 7.10.1 Navigating across documents
      2. 7.10.2 Page load processing model for HTML files
      3. 7.10.3 Page load processing model for XML files
      4. 7.10.4 Page load processing model for text files
      5. 7.10.5 Page load processing model for multipart/x-mixed-replace resources
      6. 7.10.6 Page load processing model for media
      7. 7.10.7 Page load processing model for content that uses plugins
      8. 7.10.8 Page load processing model for inline content that doesn't have a DOM
      9. 7.10.9 Navigating to a fragment
      10. 7.10.10 History traversal
          1. 7.10.10.1 Persisted user state restoration
          2. 7.10.10.2 The PopStateEvent interface
          3. 7.10.10.3 The HashChangeEvent interface
          4. 7.10.10.4 The PageTransitionEvent interface
      11. 7.10.11 Loading documents
      12. 7.10.12 Unloading documents
          1. 7.10.12.1 The BeforeUnloadEvent interface
      13. 7.10.13 Aborting a document load
      14. 7.10.14 The `X-Frame-Options` header

7.10 Browsing the web

7.10.1 Navigating across documents

Certain actions cause the browsing context to navigate to a new resource. A user agent may provide various ways for the user to explicitly cause a browsing context to navigate, in addition to those defined in this specification.

For example, following a hyperlink, form submission, and the window.open() and location.assign() methods can all cause a browsing context to navigate.

A resource has a URL, but that might not be the only information necessary to identify it. For example, a form submission that uses HTTP POST would also have the HTTP method and payload. Similarly, an iframe srcdoc document needs to know the data it is to use.

Much of the navigation process is concerned with determining how to create a new Document, which ultimately happens in the create and initialize a Document object algorithm. The parameters to this algorithm are tracked via a navigation params struct, which has the following items:

request

null or a request that started the navigation

response

a response that ultimately was navigated to (potentially a network error)

origin

an origin to use for the new Document

final sandboxing flag set

a sandboxing flag set to impose on the new Document

cross-origin opener policy

a cross-origin opener policy to use for the new Document

reserved environment

null or an environment reserved for the new Document

browsing context

the browsing context to be navigated (but see below)

browsing context switch needed

a boolean indicating whether or not the navigation should conclude by discarding the given browsing context and creating a new one

history handling

a history handling behavior

Once a navigation params struct is created, this standard does not mutate any of its items. They are only passed onward to other algorithms.

After Document creation, the session history gets updated. A history handling behavior is used to track the desired type of session history update throughout the navigation process. It is one of the following:

"default"

A regular navigation which adds a new entry to the session history.

"entry update"

A navigation to an existing session history entry to recreate that entry's Document, which was previously discarded.

"reload"

A navigation intended to reload the current page and replace the current session history entry.

"replace"

A non-reload navigation that will replace the current session history entry.

Navigation always involves source browsing context, which is the browsing context which was responsible for starting the navigation.

As explained in issue #1130 the use of a browsing context as source might not be the correct architecture.

To navigate a browsing context browsingContext to a resource resource, with an optional boolean exceptionsEnabled (default false) and an optional history handling behavior historyHandling (default "default"):

1. If resource is a URL, then set resource to a new request whose url is resource.

2. If resource is a request and historyHandling is "reload", then set resource's reload-navigation flag.

3. If the source browsing context is not allowed to navigate browsingContext, then:

   1. If exceptionsEnabled is given and is true, then throw a "SecurityError" DOMException.

   2. Otherwise, the user agent may instead offer to open resource in a new top-level browsing context or in the top-level browsing context of the source browsing context, at the user's option, in which case the user agent must navigate that designated top-level browsing context to resource as if the user had requested it independently.

      Doing so, however, can be dangerous, as it means that the user is overriding the author's explicit request to sandbox the content.

4. If there is a preexisting attempt to navigate browsingContext, and the source browsing context is the same as browsingContext, and that attempt is currently running the unload a document algorithm, then return without affecting the preexisting attempt to navigate browsingContext.

5. If the prompt to unload algorithm is being run for the active document of browsingContext, then return without affecting the prompt to unload algorithm.

6. If historyHandling is not "reload", resource is a request, resource's url equals browsingContext's active document's URL with exclude fragments flag set, and resource's url's fragment is non-null, then:

   1. Navigate to that fragment given historyHandling.

   2. Return.

7. Let activeDocumentNavigationOrigin be the origin of the active document of browsingContext.

8. Let incumbentNavigationOrigin be the origin of the incumbent settings object, or if no script was involved, the origin of the node document of the element that initiated the navigation.

9. Cancel any preexisting but not yet mature attempt to navigate browsingContext, including canceling any instances of the fetch algorithm started by those attempts. If one of those attempts has already created and initialized a new Document object, abort that Document also. (Navigation attempts that have matured already have session history entries, and are therefore handled during the update the session history with the new page algorithm, later.)

10. Prompt to unload the active document of browsingContext. If the user refused to allow the document to be unloaded, then return.

    If this instance of the navigation algorithm gets canceled while this step is running, the prompt to unload algorithm must nonetheless be run to completion.

11. Abort the active document of browsingContext.

12. If browsingContext is a child browsing context, then put it in the delaying load events mode.

    The user agent must take this child browsing context out of the delaying load events mode when this navigation algorithm later matures, or when it terminates (whether due to having run all the steps, or being canceled, or being aborted), whichever happens first.

13. Let navigationType be "form-submission" if the navigation algorithm was invoked as a result of the form submission algorithm, and "other" otherwise.

14. Let sandboxFlags be the result of determining the creation sandboxing flags given browsingContext and browsingContext's container.

15. Return to whatever algorithm invoked the navigation steps and continue running these steps in parallel.

16. This is the step that attempts to obtain resource, if necessary. Jump to the first appropriate substep:

    If resource is a response

    1. Assert: browsingContext is not a top-level browsing context.

    2. Let finalSandboxFlags be the union of browsingContext's sandboxing flags and resource's forced sandboxing flag set.

    3. Let responseOrigin be the result of determining the origin given browsingContext, resource's url, finalSandboxFlags, incumbentNavigationOrigin, and activeDocumentNavigationOrigin.

    4. Let navigationParams be a new navigation params whose request is null, response is resource, origin is responseOrigin, final sandboxing flag set is finalSandboxFlags, cross-origin opener policy is "unsafe-none", reserved environment is null, browsing context is browsingContext, browsing context switch needed is false, and history handling is historyHandling.

    5. Run process a navigate response with navigationType, the source browsing context, and navigationParams.

    If resource is a request whose url's scheme is "javascript"

    Queue a global task on the DOM manipulation task source given browsingContext's active window to run these steps:

    1. Let response be the result of executing a javascript: URL request given resource, the source browsing context, and browsingContext.

    2. Let finalSandboxFlags be the union of browsingContext's sandboxing flags and response's forced sandboxing flag set.

    3. Let navigationParams be a new navigation params whose request is resource, response is response, origin is activeDocumentNavigationOrigin, final sandboxing flag set is finalSandboxFlags, cross-origin opener policy is browsingContext's active document's cross-origin opener policy, reserved environment is null, browsing context is browsingContext, browsing context switch needed is false, and history handling is historyHandling.

    4. Run process a navigate response with navigationType, the source browsing context, and navigationParams.

    So for example a javascript: URL in an href attribute of an a element would only be evaluated when the link was followed, while such a URL in the src attribute of an iframe element would be evaluated in the context of the iframe's nested browsing context when the iframe is being set up. Once evaluated, its return value (if it was a string) would replace that browsing context's active document, thus also changing the corresponding Window object.

    If resource is to be fetched using `GET`, and there are relevant application caches that are identified by a URL with the same origin as the URL in question, and that have this URL as one of their entries, excluding entries marked as foreign, and whose mode is fast, and the user agent is not in a mode where it will avoid using application caches

    Fetch resource from the most appropriate application cache of those that match.

    For example, imagine an HTML page with an associated application cache displaying an image and a form, where the image is also used by several other application caches. If the user right-clicks on the image and chooses "View Image", then the user agent could decide to show the image from any of those caches, but it is likely that the most useful cache for the user would be the one that was used for the aforementioned HTML page. On the other hand, if the user submits the form, and the form does a POST submission, then the user agent will not use an application cache at all; the submission will be made to the network.

    This still needs to be integrated with the Fetch standard. [FETCH]

    If resource is a request whose url's scheme is a fetch scheme

    Run process a navigate fetch given resource, the source browsing context, browsingContext, navigationType, sandboxFlags, incumbentNavigationOrigin, activeDocumentNavigationOrigin, and historyHandling.

    Otherwise, resource is a request whose url's scheme is neither "javascript" nor a fetch scheme

    Run process a navigate URL scheme given resource's url and browsingContext.

To process a navigate fetch, given a request request, two browsing contexts sourceBrowsingContext and browsingContext, a string navigationType, a sandboxing flag set sandboxFlags, two origins incumbentNavigationOrigin and activeDocumentNavigationOrigin, and a history handling behavior historyHandling:

1. Let response be null.

2. Set request's client to sourceBrowsingContext's active document's relevant settings object, destination to "document", mode to "navigate", credentials mode to "include", use-URL-credentials flag, redirect mode to "manual", and replaces client id to browsingContext's active document's relevant settings object's id.

3. If browsingContext's container is non-null:

   1. If the browsingContext's container has a browsing context scope origin, then set request's origin to that browsing context scope origin.

   2. Set request's destination to browsingContext's container's local name.

4. Let reservedEnvironment be null.

5. Let responseOrigin be null.

6. Let coopEnforcementResult be a new cross-origin opener policy enforcement result whose needs a browsing context group switch is false, origin is browsingContext's active document's origin, and cross-origin opener policy is browsingContext's active document's cross-origin opener policy.

7. Let finalSandboxFlags be an empty sandboxing flag set.

8. Let responseCOOP be "unsafe-none".

9. While true:

   1. Let currentURL be response's location URL, if response is not null, and request's current URL otherwise.

   2. If reservedEnvironment is not null and currentURL's origin is not the same as reservedEnvironment's creation URL's origin, then:

      1. Run the environment discarding steps for reservedEnvironment.

      2. Set reservedEnvironment to null.

   3. If reservedEnvironment is null, then:

      1. Let topLevelCreationURL be currentURL.

      2. Let topLevelOrigin be null.

      3. If browsingContext is not a top-level browsing context, then:

         1. Let parentEnvironment be browsingContext's container's relevant settings object.

         2. Set topLevelCreationURL to parentEnvironment's top-level creation URL and topLevelOrigin to parentEnvironment's top-level origin.

      4. Set reservedEnvironment to a new environment whose id is a unique opaque string, target browsing context is browsingContext, creation URL is currentURL, top-level creation URL is topLevelCreationURL, and top-level origin is topLevelOrigin.

         The created environment's active service worker is set in the Handle Fetch algorithm during the fetch if the request URL matches a service worker registration. [SW]

   4. Set request's reserved client to reservedEnvironment.

   5. If the Should navigation request of type from source in target be blocked by Content Security Policy? algorithm returns "Blocked" when executed upon request, navigationType, sourceBrowsingContext, and browsingContext, then set response to a network error and break. [CSP]

   6. Otherwise:

      1. If response is null, fetch request.

      2. Otherwise, perform HTTP-redirect fetch using request and response.

      3. Wait for the task on the networking task source to process response and set response to the result.

      4. Set finalSandboxFlags to the union of browsingContext's sandboxing flags and response's forced sandboxing flag set.

      5. Set responseOrigin to the result of determining the origin given browsingContext, request's url, finalSandboxFlags, incumbentNavigationOrigin, and activeDocumentNavigationOrigin.

      6. If browsingContext is a top-level browsing context, then:

         1. Set responseCOOP to the result of obtaining a cross-origin opener policy given response and reservedEnvironment.

         2. If sandboxFlags is not empty and responseCOOP is not "unsafe-none", then set response to an appropriate network error and break.

            This results in a network error as one cannot simultaneously provide a clean slate to a response using cross-origin opener policy and sandbox the result of navigating to that response.

         3. Set coopEnforcementResult to the result of enforcing the response's cross-origin opener policy given browsingContext, responseOrigin, responseCOOP, and coopEnforcementResult.

      7. If response is not a network error, browsingContext is a child browsing context, and the result of performing a cross-origin resource policy check with browsingContext's container document's origin, browsingContext's container document's relevant settings object, request's destination, response, and true is blocked, then set response to a network error and break.

         Here we're running the cross-origin resource policy check against the parent browsing context rather than sourceBrowsingContext. This is because we care about the same-originness of the embedded content against the parent context, not the navigation source.

      8. If response does not have a location URL or the location URL is not a URL whose scheme is an HTTP(S) scheme, then break.

         Navigation handles redirects manually as navigation is the only place in the web platform that cares for redirects to mailto: URLs and such.

10. If response has a location URL that is failure, then set response to a network error.

11. Otherwise, if response has a location URL that is a URL whose scheme is "blob", "file", "filesystem", or "javascript", then set response to a network error.

12. Otherwise, if response has a location URL that is a URL whose scheme is a fetch scheme, then run process a navigate fetch with a new request whose url is response's location URL, sourceBrowsingContext, browsingContext, navigationType, sandboxFlags, incumbentNavigationOrigin, activeDocumentNavigationOrigin, and historyHandling, and return.

13. Otherwise, if response has a location URL that is a URL, run the process a navigate URL scheme given response's location URL and browsingContext, and return.

14. Fallback in prefer-online mode: If response was not fetched from an application cache, and was to be fetched using `GET`, and there are relevant application caches that are identified by a URL with the same origin as the URL in question, and that have this URL as one of their entries, excluding entries marked as foreign, and whose mode is prefer-online, and the user didn't cancel the navigation attempt during the earlier step, and response is either a network error or its status is not an ok status, then:

    Let candidate be the response identified by the URL in question from the most appropriate application cache of those that match.

    If candidate is not marked as foreign, then the user agent must discard the failed load and instead continue along these steps using candidate as response. The user agent may indicate to the user that the original page load failed, and that the page used was a previously cached response.

15. Fallback for fallback entries: If response was not fetched from an application cache, and was to be fetched using `GET`, and its URL matches the fallback namespace of one or more relevant application caches, and the most appropriate application cache of those that match does not have an entry in its online safelist that has the same origin as response's URL and that is a prefix match for response's URL, and the user didn't cancel the navigation attempt during the earlier step, and response is either a network error or its status is not an ok status, then:

    Let candidate be the fallback response specified for the fallback namespace in question. If multiple application caches match, the user agent must use the fallback of the most appropriate application cache of those that match.

    If candidate is not marked as foreign, then the user agent must discard the failed load and instead continue along these steps using candidate as response. The document's URL, if appropriate, will still be the originally requested URL, not the fallback URL, but the user agent may indicate to the user that the original page load failed, that the page used was a fallback response, and what the URL of the fallback response actually is.

16. Let navigationParams be a new navigation params whose request is request, response is response, origin is responseOrigin, final sandboxing flag set is finalSandboxFlags, cross-origin opener policy is responseCOOP, reserved environment is reservedEnvironment, browsing context is browsingContext, browsing context switch needed is coopEnforcementResult's needs a browsing context group switch, and history handling is historyHandling.

17. Run process a navigate response with navigationType, the source browsing context, and navigationParams.

To process a navigate response, given a string navigationType, a browsing context source, and a navigation params navigationParams:

1. Let response be navigationParams's response.

2. Let browsingContext be navigationParams's browsing context.

3. Let failure be false.

4. If response is a network error, then set failure to true.

   Otherwise, if the result of should navigation response to navigation request of type from source in target be blocked by Content Security Policy? given navigationParams's request, response, navigationType, source, and browsingContext is "Blocked", then set failure to true. [CSP]

   Otherwise, if the result of checking a navigation response's adherence to its embedder policy given response and browsingContext is false, then set failure to true.

   Otherwise, if the result of checking a navigation response's adherence to `X-Frame-Options` given response, browsingContext, and navigationParams's origin is false, then set failure to true.

5. If failure is true, then:

   1. Display the inline content with an appropriate error shown to the user given browsingContext.

   2. Run the environment discarding steps for navigationParams's reserved environment.

   3. Return.

   This is where the network errors defined and propagated by Fetch, such as DNS or TLS errors, end up being displayed to users. [FETCH]

6. If response's status is 204 or 205, then return.

7. If response has a `Content-Disposition` header specifying the attachment disposition type, then:

   1. If the result of running the allowed to download given source and browsingContext is true, then handle response as a download.

   2. Return.

8. Let type be the computed type of response.

9. If the user agent has been configured to process resources of the given type using some mechanism other than rendering the content in a browsing context, then skip this step. Otherwise, if the type is one of the following types, jump to the appropriate entry in the following list, and process response as described there:

   an HTML MIME type

   Follow the steps given in the HTML document section providing navigationParams. Once the steps have completed, return.

   an XML MIME type that is not an explicitly supported XML MIME type

   Follow the steps given in the XML document section providing navigationParams and type. Once the steps have completed, return.

   a JavaScript MIME type

   a JSON MIME type that is not an explicitly supported JSON MIME type

   "text/cache-manifest"

   "text/css"

   "text/plain"

   "text/vtt"

   Follow the steps given in the plain text file section providing navigationParams and type. Once the steps have completed, return.

   "multipart/x-mixed-replace"

   Follow the steps given in the multipart/x-mixed-replace section providing navigationParams. Once the steps have completed, return.

   A supported image, video, or audio type

   Follow the steps given in the media section providing navigationParams and type. Once the steps have completed, return.

   A type that will use an external application to render the content in browsingContext

   Follow the steps given in the plugin section providing navigationParams and type. Once the steps have completed, return.

   An explicitly supported XML MIME type is an XML MIME type for which the user agent is configured to use an external application to render the content (either a plugin rendering directly in browsingContext, or a separate application), or one for which the user agent has dedicated processing rules (e.g., a web browser with a built-in Atom feed viewer would be said to explicitly support the application/atom+xml MIME type), or one for which the user agent has a dedicated handler.

   An explicitly supported JSON MIME type is a JSON MIME type for which the user agent is configured to use an external application to render the content (either a plugin rendering directly in browsingContext, or a separate application), or one for which the user agent has dedicated processing rules, or one for which the user agent has a dedicated handler.

10. Non-document content: If, given type, the new resource is to be handled by displaying some sort of inline content, e.g., a native rendering of the content or an error message because the specified type is not supported, then display the inline content given browsingContext, and then return.

11. Otherwise, the document's type is such that the resource will not affect browsingContext, e.g., because the resource is to be handed to an external application or because it is an unknown type that will be processed as a download. Process the resource appropriately.

To process a navigate URL scheme, given a URL url and browsing context browsingContext, run these steps:

1. If url is to be handled using a mechanism that does not affect browsingContext, e.g., because url's scheme is handled externally, then proceed with that mechanism instead.

2. Otherwise, url is to be handled by displaying some sort of inline content, e.g., an error message because the specified scheme is not one of the supported protocols, or an inline prompt to allow the user to select a registered handler for the given scheme. Display the inline content given browsingContext.

   In the case of a registered handler being used, navigate will be invoked with a new URL.

When a resource is handled by passing its URL or data to an external software package separate from the user agent (e.g. handing a mailto: URL to a mail client, or a Word document to a word processor), user agents should attempt to mitigate the risk that this is an attempt to exploit the target software, e.g. by prompting the user to confirm that the source browsing context's active document's origin is to be allowed to invoke the specified software. In particular, if the navigate algorithm was invoked when source browsing context's active window does not have transient activation, the user agent should not invoke the external software package without prior user confirmation.

For example, there could be a vulnerability in the target software's URL handler which a hostile page would attempt to exploit by tricking a user into clicking a link.

To execute a javascript: URL request, given a request request and two browsing contexts source and browsingContext, run these steps:

1. Let response be a response whose status is 204.

2. If both of the following are true:

   • source's active document's origin is same origin with browsingContext's active document's origin.

     As explained in issue #2591 this step does not work and presents a security issue.

   • The Should navigation request of type from source in target be blocked by Content Security Policy? algorithm returns "Allowed" when executed upon request, "other", source, and browsingContext. [CSP]

   then:

   1. Let urlString be the result of running the URL serializer on request's url.

   2. Let encodedScriptSource be the result of removing the leading "javascript:" from urlString.

   3. Let scriptSource be the UTF-8 decoding of the percent-decoding of encodedScriptSource.

   4. Append browsingContext's active document's URL to request's URL list.

   5. Let settings be browsingContext's active document's relevant settings object.

   6. Let baseURL be settings's API base URL.

   7. Let script be the result of creating a classic script given scriptSource, settings, baseURL, and the default classic script fetch options.

   8. Let evaluationStatus be the result of running the classic script script.

   9. Let result be undefined if evaluationStatus is an abrupt completion or evaluationStatus.[[Value]] is empty, or evaluationStatus.[[Value]] otherwise.

   10. If Type(result) is String, then set response to a response whose header list consists of `Content-Type`/`text/html` and `Referrer-Policy`/settings's referrer policy, and whose body is result.

       The exact conversion between the string result and the bytes that comprise a response body is not yet specified, pending further investigation into user agent behavior. See issue #1129.

3. Return response.

In addition to the specific issues linked above, javascript: URLs have a dedicated label on the issue tracker documenting various problems with their specification.

Some of the sections below, to which the above algorithm defers in certain cases, use the following steps to create and initialize a Document object, given a type type, content type contentType, and navigation params navigationParams:

1. Let browsingContext be navigationParams's browsing context.

2. If navigationParams's browsing context switch needed is true, then set browsingContext to the result of the obtain a browsing context to use for a navigation response algorithm, given browsingContext, navigationParams's final sandboxing flag set, and navigationParams's cross-origin opener policy.

3. Let permissionsPolicy be the result of creating a permissions policy from a response given browsingContext, navigationParams's origin, and navigationParams's response. [PERMISSIONSPOLICY]

   The creating a permissions policy from a response algorithm makes use of the passed origin. If document.domain has been used for browsingContext's container document, then its origin cannot be same origin-domain with the passed origin, because these steps run before the document is created, so it cannot itself yet have used document.domain. Note that this means that Permissions Policy checks are less permissive compared to doing a same origin check instead.

   See below for some examples of this in action.

4. Let creationURL be navigationParams's response's URL.

5. If navigationParams's request is non-null, then set creationURL to navigationParams's request's current URL.

6. If browsingContext's only entry in its session history is the initial about:blank Document that was added when that browsing context was created, and navigationParams's history handling is "replace", and that Document's origin is same origin-domain with navigationParams's origin, then do nothing.

7. Otherwise:

   1. Let oiHeader be the result of getting a structured field value given `Origin-Isolation` and "item" from response's header list.

   2. Let requestsOI be true if oiHeader is not null and oiHeader[0] is the boolean true; otherwise false.

   3. If reservedEnvironment is a non-secure context, then set requestsOI to false.

   4. Let agent be the result of obtaining a similar-origin window agent given navigationParams's origin, browsingContext's group, and requestsOI.

   5. Let realm execution context be the result of creating a new JavaScript realm given agent and the following customizations:

      • For the global object, create a new Window object.

      • For the global this binding, use browsingContext's WindowProxy object.

   6. Let topLevelCreationURL be creationURL.

   7. Let topLevelOrigin be navigationParams's origin.

   8. If browsingContext is not a top-level browsing context, then:

      1. Let parentEnvironment be browsingContext's container's relevant settings object.

      2. Set topLevelCreationURL to parentEnvironment's top-level creation URL.

      3. Set topLevelOrigin to parentEnvironment's top-level origin.

   9. Set up a window environment settings object with realm execution context, navigationParams's reserved environment, topLevelCreationURL, and topLevelOrigin.

8. Let document be a new Document, whose type is type, content type is contentType, origin is navigationParams's origin, permissions policy is permissionsPolicy, active sandboxing flag set is navigationParams's final sandboxing flag set, and cross-origin opener policy is navigationParams's cross-origin opener policy.

9. Set document's URL to creationURL.

10. Set document's referrer policy to the result of parsing the `Referrer-Policy` header of navigationParams's response. [REFERRERPOLICY]

11. Set document's embedder policy to the result of obtaining an embedder policy from navigationParams's response.

12. Initialize a Document's CSP list given document, navigationParams's response, and navigationParams's request. [CSP]

13. If navigationParams's request is non-null, then:

    1. Set document's referrer to the empty string.

    2. Let referrer be navigationParams's request's referrer.

    3. If referrer is a URL record, then set document's referrer to the serialization of referrer.

       Per Fetch, referrer will be either a URL record or "no-referrer" at this point.

14. If navigationParams's response has a `Refresh` header, then:

    1. Let value be the isomorphic decoding of the value of the header.

    2. Run the shared declarative refresh steps with document and value.

    We do not currently have a spec for how to handle multiple `Refresh` headers. This is tracked as issue #2900.

15. Return document.

<!-- https://foo.example.com/a.html -->
<!doctype html>
<script>
document.domain = 'example.com';
</script>
<iframe src=b.html></iframe>

<!-- https://bar.example.com/b.html -->
<!doctype html>
<script>
document.domain = 'example.com'; // This happens after the document is initialized
new PaymentRequest(…); // Not allowed to use
</script>

<!-- https://example.com/a.html -->
<!doctype html>
<iframe src=b.html></iframe>
<!-- The child document is now initialized, before the script below is run. -->
<script>
document.domain = 'example.com';
</script>

<!-- https://example.com/b.html -->
<!doctype html>
<script>
new PaymentRequest(…); // Allowed to use
</script>

Some of the sections below, to which the above algorithm defers in certain cases, require the user agent to update the session history with the new page, given some navigation params navigationParams. When a user agent is required to do this, it must queue a global task on the networking task source, given the relevant global object of the Document object of the current entry (not the new one), to run the following steps:

1. Unload the Document object of the current entry.

   If this instance of the navigation algorithm is canceled while this step is running the unload a document algorithm, then the unload a document algorithm must be allowed to run to completion, but this instance of the navigation algorithm must not run beyond this step. (In particular, for instance, the cancelation of this algorithm does not abort any event dispatch or script execution occurring as part of unloading the document or its descendants.)

2. If navigationParams's history handling is "reload" or "entry update"

   1. Replace the Document of the entry being updated, and any other entries that referenced the same document as that entry, with the new Document.

   2. Traverse the history to the new entry.

   If the navigation was initiated with a URL that equals the browsing context's active document's URL

   1. Replace the current entry with a new entry representing the new resource and its Document object, related state, and the default scroll restoration mode of "auto".

   2. Traverse the history to the new entry.

   Otherwise

   1. Remove all the entries in the browsing context's session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

      This doesn't necessarily have to affect the user agent's user interface.

   2. Append a new entry at the end of the History object representing the new resource and its Document object, related state, and the default scroll restoration mode of "auto".

   3. Traverse the history to the new entry, with historyHandling set to navigationParams's history handling.

3. The navigation algorithm has now matured.

4. Try to scroll to the fragment for the Document.

To try to scroll to the fragment for a Document document, perform the following steps in parallel:

1. Wait for an implementation-defined amount of time. (This is intended to allow the user agent to optimize the user experience in the face of performance concerns.)

2. Queue a global task on the networking task source given document's relevant global object to run these steps:

   1. If document has no parser, or its parser has stopped parsing, or the user agent has reason to believe the user is no longer interested in scrolling to the fragment, then abort these steps.

   2. Scroll to the fragment given in document's URL. If this does not find an indicated part of the document, then try to scroll to the fragment for document.

7.10.2 Page load processing model for HTML files

When an HTML document is to be loaded, given navigation params navigationParams, the user agent must queue a task on the networking task source to:

1. Let document be the result of creating and initializing a Document object given "html", "text/html", and navigationParams.

2. Create an HTML parser and associate it with the document. Each task that the networking task source places on the task queue while fetching runs must then fill the parser's input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

   The input byte stream converts bytes into characters for use in the tokenizer. This process relies, in part, on character encoding information found in the real Content-Type metadata of the resource; the computed type is not used for this purpose.

When no more bytes are available, the user agent must queue a global task on the networking task source given the newly-created Document's relevant global object for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but before any script execution, certainly before the parser stops, the user agent must update the session history with the new page given navigationParams.

Application cache selection happens in the HTML parser.

7.10.3 Page load processing model for XML files

When faced with displaying an XML file inline, provided navigation params navigationParams and a string type, user agents must follow the requirements defined in XML and Namespaces in XML, XML Media Types, DOM, and other relevant specifications to create and initialize a Document object given "xml", type, and navigationParams. They must also create a corresponding XML parser. [XML] [XMLNS] [RFC7303] [DOM]

At the time of writing, the XML specification community had not actually yet specified how XML and the DOM interact.

The actual HTTP headers and other metadata, not the headers as mutated or implied by the algorithms given in this specification, are the ones that must be used when determining the character encoding according to the rules given in the above specifications. Once the character encoding is established, the document's character encoding must be set to that character encoding.

If the document element, as parsed according to XML cited above, is found to be an html element with an attribute manifest whose value is not the empty string, then, as soon as the element is inserted into the document, the user agent must parse the value of that attribute relative to that element's node document, and if that is successful, must apply the URL serializer algorithm to the resulting URL record with the exclude fragment flag set to obtain manifest URL, and then run the application cache selection algorithm with manifest URL as the manifest URL, passing in the newly-created Document. Otherwise, if the attribute is absent, its value is the empty string, or parsing its value fails, then as soon as the document element is inserted into the document, the user agent must run the application cache selection algorithm with no manifest, and passing in the Document.

Because the processing of the manifest attribute happens only once the document element is parsed, any URLs referenced by processing instructions before the document element (such as <?xml-stylesheet?> PIs) will be fetched from the network and cannot be cached.

Then, with the newly created Document, the user agent must update the session history with the new page given navigationParams. User agents may do this before the complete document has been parsed (thus achieving incremental rendering), and must do this before any scripts are to be executed.

Error messages from the parse process (e.g., XML namespace well-formedness errors) may be reported inline by mutating the Document.

7.10.4 Page load processing model for text files

When a plain text document is to be loaded, provided navigation params navigationParams and a string type, the user agent must queue a task on the networking task source to:

1. Let document be the result of creating and initializing a Document object given "html", type, and navigationParams.

2. Create an HTML parser and associate it with the document. Act as if the tokenizer had emitted a start tag token with the tag name "pre" followed by a single U+000A LINE FEED (LF) character, and switch the HTML parser's tokenizer to the PLAINTEXT state. Each task that the networking task source places on the task queue while fetching runs must then fill the parser's input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

The rules for how to convert the bytes of the plain text document into actual characters, and the rules for actually rendering the text to the user, are defined by the specifications for the computed MIME type of the resource (i.e., type).

The document's character encoding must be set to the character encoding used to decode the document.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

When no more bytes are available, the user agent must queue a global task on the networking task source given the newly-created Document's relevant global object for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but potentially before the page has finished parsing, the user agent must update the session history with the new page given navigationParams.

User agents may add content to the head element of the Document, e.g., linking to a style sheet, providing script, or giving the document a title.

In particular, if the user agent supports the Format=Flowed feature of RFC 3676 then the user agent would need to apply extra styling to cause the text to wrap correctly and to handle the quoting feature. This could be performed using, e.g., a CSS extension.

7.10.5 Page load processing model for multipart/x-mixed-replace resources

When a resource with the type multipart/x-mixed-replace is to be loaded in a browsing context, the user agent must parse the resource using the rules for multipart types. [RFC2046]

This algorithm is passed navigation params, but it's unclear how exactly to use them.

For each body part obtained from the resource, the user agent must run process a navigate response using the new body part and the same browsing context, with history handling set to "replace" if a previous body part from the same resource resulted in a creating and initializing a Document object, and otherwise using the same setup as the navigate attempt that caused this section to be invoked in the first place.

For the purposes of algorithms processing these body parts as if they were complete stand-alone resources, the user agent must act as if there were no more bytes for those resources whenever the boundary following the body part is reached.

Thus, load events (and for that matter unload events) do fire for each body part loaded.

7.10.6 Page load processing model for media

When an image, video, or audio resource is to be loaded, provided navigation params navigationParams and a string type, the user agent should:

1. Let document be the result of creating and initializing a Document object given "html", type, and navigationParams.

2. Append an html element to document.

3. Append a head element to the html element.

4. Append a body element to the html element.

5. Append an element host element for the media, as described below, to the body element.

6. Set the appropriate attribute of the element host element, as described below, to the address of the image, video, or audio resource.

The element host element to create for the media is the element given in the table below in the second cell of the row whose first cell describes the media. The appropriate attribute to set is the one given by the third cell in that same row.

Then, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page given navigationParams.

User agents may add content to the head element of the Document, or attributes to the element host element, e.g., to link to a style sheet, to provide a script, to give the document a title, or to make the media autoplay.

7.10.7 Page load processing model for content that uses plugins

When a resource that requires an external resource to be rendered is to be loaded, provided navigation params navigationParams and a string type, the user agent should:

1. Let document be the result of creating and initializing a Document object given "html", type, and navigationParmas.

2. Mark document as being a plugin document

3. Append an html element to document.

4. Append a head element to the html element.

5. Append a body element to the html element.

6. Append an embed to the body element

7. Set the src attribute of the embed element to the address of the resource.

The term plugin document is used by Content Security Policy as part of the mechanism that ensures iframes can't be used to evade plugin-types directives. [CSP]

Then, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page given navigationParams.

User agents may add content to the head element of the Document, or attributes to the embed element, e.g. to link to a style sheet or to give the document a title.

If the Document's active sandboxing flag set has its sandboxed plugins browsing context flag set, the synthesized embed element will fail to render the content if the relevant plugin cannot be secured.

7.10.8 Page load processing model for inline content that doesn't have a DOM

When the user agent is to display a user agent page inline, provided a browsing context browsingContext, the user agent should:

1. Let navigationParams be a new navigation params whose request is null, response is null, origin is a new opaque origin, final sandboxing flag set is an empty set, cross-origin opener policy is "unsafe-none", reserved environment is null, browsing context is browsingContext, and browsing context switch needed is false.

   The algorithm called in the next step is not prepared to deal with a null response. Probably we should synthesize one instead.

2. Let document be the result of creating and initializing a Document object given "html", "text/html", and navigationParams.

3. Either associate document with a custom rendering that is not rendered using the normal Document rendering rules, or mutate document until it represents the content the user agent wants to render.

Once the page has been set up, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, passing in the newly-created Document.

After creating the Document object, but potentially before the page has been completely set up, the user agent must update the session history with the new page given navigationParams.

7.10.9 Navigating to a fragment

To navigate to a fragment, with a history handling behavior historyHandling:

1. If historyHandling is not "replace", then remove all the entries in the browsing context's session history after the current entry. (If the current entry is the last entry in the session history, then no entries are removed.)

   This doesn't necessarily have to affect the user agent's user interface.

2. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.

3. Append a new entry at the end of the History object representing the new resource and its Document object, related state, and current entry's scroll restoration mode. Its URL must be set to the address to which the user agent was navigating. The title must be left unset.

4. Traverse the history to the new entry, with historyHandling set to historyHandling and with nonBlockingEvents set to true. This will scroll to the fragment given in what is now the document's URL.

If the scrolling fails because the relevant ID has not yet been parsed, then the original navigation algorithm will take care of the scrolling instead, as the last few steps of its update the session history with the new page algorithm.

When the user agent is required to scroll to the fragment and the indicated part of the document, if any, is being rendered, the user agent must either change the scrolling position of the document using the following algorithm, or perform some other action such that the indicated part of the document is brought to the user's attention. If there is no indicated part, or if the indicated part is not being rendered, then the user agent must do nothing. The aforementioned algorithm is as follows:

1. If there is no indicated part of the document, set the Document's target element to null.

2. If the indicated part of the document is the top of the document, then:

   1. Set the Document's target element to null.

   2. Scroll to the beginning of the document for the Document. [CSSOMVIEW]

3. Otherwise:

   1. Let target be element that is the indicated part of the document.

   2. Set the Document's target element to target.

   3. Scroll target into view, with behavior set to "auto", block set to "start", and inline set to "nearest". [CSSOMVIEW]

   4. Run the focusing steps for target, with the Document's viewport as the fallback target.

   5. Move the sequential focus navigation starting point to target.

The indicated part of the document is the one that the fragment, if any, identifies. The semantics of the fragment in terms of mapping it to a node is defined by the specification that defines the MIME type used by the Document (for example, the processing of fragments for XML MIME types is the responsibility of RFC7303). [RFC7303]

There is also a target element for each Document, which is used in defining the :target pseudo-class and is updated by the above algorithm. It is initially null.

For HTML documents (and HTML MIME types), the following processing model must be followed to determine what the indicated part of the document is.

1. Let fragment be the document's URL's fragment.

2. If fragment is the empty string, then the indicated part of the document is the top of the document; return.

3. If find a potential indicated element with fragment returns non-null, then the return value is the indicated part of the document; return.

4. Let fragmentBytes be the result of percent-decoding fragment.

5. Let decodedFragment be the result of running UTF-8 decode without BOM on fragmentBytes.

6. If find a potential indicated element with decodedFragment returns non-null, then the return value is the indicated part of the document; return.

7. If decodedFragment is an ASCII case-insensitive match for the string top, then the indicated part of the document is the top of the document; return.

8. There is no indicated part of the document.

To find a potential indicated element given a string fragment, run these steps:

1. If there is an element in the document tree that has an ID equal to fragment, then return the first such element in tree order.

2. If there is an a element in the document tree that has a name attribute whose value is equal to fragment, then return the first such element in tree order.

3. Return null.

7.10.10 History traversal

To traverse the history to a session history entry entry, with an optional history handling behavior historyHandling (default "default"), an optional boolean nonBlockingEvents (default false), and an optional boolean explicitHistoryNavigation (default false):

This algorithm is not just invoked when explicitly going back or forwards in the session history — it is also invoked in other situations, for example when navigating a browsing context, as part of updating the session history with the new page.

1. If entry no longer holds a Document object, then:

   1. Let request be a new request whose url is entry's URL.

   2. If explicitHistoryNavigation is true, then set request's history-navigation flag.

   3. Assert: historyHandling is not "replace".

   4. Navigate the browsing context to request with historyHandling set to "entry update". The navigation must be done using the same source browsing context as was used the first time entry was created.

      The "navigate" algorithm reinvokes this "traverse" algorithm to complete the traversal, at which point entry holds a Document object.

      If the resource was obtained using a non-idempotent action, for example a POST form submission, or if the resource is no longer available, for example because the computer is now offline and the page wasn't cached, navigating to it again might not be possible. In this case, the navigation will result in a different page than previously; for example, it might be an error message explaining the problem or offering to resubmit the form.

   5. Return.

2. If the current entry's title was not set by the pushState() or replaceState() methods, then set its title to the value returned by the document.title IDL attribute.

3. If appropriate, update the current entry in the browsing context's Document object's History object to reflect any state that the user agent wishes to persist. The entry is then said to be an entry with persisted user state.

4. If entry has a different Document object than the current entry, then run the following substeps:

   1. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.

   2. If the origin of entry's Document object is not the same as the origin of the current entry's Document object, then run the following subsubsteps:

      1. The current browsing context name must be stored with all the entries in the history that are associated with Document objects with the same origin as the active document and that are contiguous with the current entry.

      2. If the browsing context is a top-level browsing context, but not an auxiliary browsing context, then set the browsing context's name to the empty string.

   3. Set the active document of the browsing context to entry's Document object.

   4. If entry has a browsing context name, then run the following subsubsteps:

      1. Set the browsing context's browsing context name to entry's browsing context name.

      2. Clear any browsing context names of all entries in the history that are associated with Document objects with the same origin as the new active document and that are contiguous with entry.

   5. If entry's Document object has any form controls whose autofill field name is "off", invoke the reset algorithm of each of those elements.

   6. If the current document readiness of entry's Document object is "complete", then queue a global task on the DOM manipulation task source given entry's Document's relevant global object to run the following subsubsteps:

      1. If the Document's page showing flag is true, then abort these steps.

      2. Set the Document's page showing flag to true.

      3. Run any session history document visibility change steps for Document that are defined by other applicable specifications.

         This is specifically intended for use by Page Visibility. [PAGEVIS]

      4. Fire an event named pageshow at the Document object's relevant global object, using PageTransitionEvent, with the persisted attribute initialized to true, and legacy target override flag set.

5. Set the document's URL to entry's URL.

6. If entry has a URL whose fragment is not identical to that of the current entry's, and the two share the same Document object, then let hash changed be true, and let old URL be the current entry's URL and new URL be entry's URL. Otherwise, let hash changed be false.

7. If historyHandling is "replace", then remove the entry immediately before the specified entry in the session history.

8. If entry is not an entry with persisted user state, but its URL's fragment is non-null, then scroll to the fragment.

9. Set the current entry to entry.

10. Let targetRealm be the current Realm Record.

11. If entry has serialized state, then let state be StructuredDeserialize(entry's serialized state, targetRealm). If this throws an exception, catch it, ignore the exception, and let state be null.

12. Otherwise, let state be null.

13. Set history.state to state.

14. Let state changed be true if entry's Document object has a latest entry, and that entry is not entry; otherwise let it be false.

15. Set entry's Document object's latest entry to entry.

16. If nonBlockingEvents is false, then run the following substeps immediately. Otherwise, queue a global task on the DOM manipulation task source given entry's Document's relevant global object to run the following substeps instead.

    1. If state changed is true, then fire an event named popstate at the Document object's relevant global object, using PopStateEvent, with the state attribute initialized to state.

    2. If entry is an entry with persisted user state, then the user agent may restore persisted user state and update aspects of the document and its rendering.

    3. If hash changed is true, then fire an event named hashchange at the browsing context's active window, using HashChangeEvent, with the oldURL attribute initialized to old URL and the newURL attribute initialized to new URL.

7.10.10.1 Persisted user state restoration

When the user agent is to restore persisted user state from a history entry, it must run the following steps immediately:

1. If the entry has a scroll restoration mode, let scrollRestoration be that. Otherwise let scrollRestoration be "auto"

2. If scrollRestoration is "manual", then the user agent should not restore the scroll position for the Document or any of its scrollable regions, with the exception of any child browsing contexts of Document's browsing context whose scroll restoration is controlled by their own history entry's scroll restoration mode, otherwise, it may do so.

3. Optionally, update other aspects of the document and its rendering, for instance values of form fields, that the user agent had previously recorded.

This can even include updating the dir attribute of textarea elements or input elements whose type attribute is in either the Text state or the Search state, if the persisted state includes the directionality of user input in such controls.

Not restoring the scroll position by user agent does not imply that the scroll position will be left at any particular value (e.g., (0,0)). The actual scroll position depends on the navigation type and the user agent's particular caching strategy. So web applications cannot assume any particular scroll position but rather are urged to set it to what they want it to be.

7.10.10.2 The PopStateEvent interface

[Exposed=Window]
interface PopStateEvent : Event {
  constructor(DOMString type, optional PopStateEventInit eventInitDict = {});

  readonly attribute any state;
};

dictionary PopStateEventInit : EventInit {
  any state = null;
};

event . state

Returns a copy of the information that was provided to pushState() or replaceState().

The state attribute must return the value it was initialized to. It represents the context information for the event, or null, if the state represented is the initial state of the Document.

7.10.10.3 The HashChangeEvent interface

[Exposed=Window]
interface HashChangeEvent : Event {
  constructor(DOMString type, optional HashChangeEventInit eventInitDict = {});

  readonly attribute USVString oldURL;
  readonly attribute USVString newURL;
};

dictionary HashChangeEventInit : EventInit {
  USVString oldURL = "";
  USVString newURL = "";
};

event . oldURL

Returns the URL of the session history entry that was previously current.

event . newURL

Returns the URL of the session history entry that is now current.

The oldURL attribute must return the value it was initialized to. It represents context information for the event, specifically the URL of the session history entry that was traversed from.

The newURL attribute must return the value it was initialized to. It represents context information for the event, specifically the URL of the session history entry that was traversed to.

7.10.10.4 The PageTransitionEvent interface

[Exposed=Window]
interface PageTransitionEvent : Event {
  constructor(DOMString type, optional PageTransitionEventInit eventInitDict = {});

  readonly attribute boolean persisted;
};

dictionary PageTransitionEventInit : EventInit {
  boolean persisted = false;
};

event . persisted

For the pageshow event, returns false if the page is newly being loaded (and the load event will fire). Otherwise, returns true.

For the pagehide event, returns false if the page is going away for the last time. Otherwise, returns true, meaning that (if nothing conspires to make the page unsalvageable) the page might be reused if the user navigates back to this page.

Things that can cause the page to be unsalvageable include:

• Listening for beforeunload events
• Listening for unload events
• Having iframes that are not salvageable
• Active WebSocket objects
• Aborting a Document

The persisted attribute must return the value it was initialized to. It represents the context information for the event.

7.10.11 Loading documents

A Document has a completely loaded time (a time or null), which is initially null.

A Document is considered completely loaded if its completely loaded time is non-null.

To completely finish loading a Document document:

1. Assert: document's browsing context is non-null.

2. Set document's completely loaded time to the current time.

3. Let container be document's browsing context's container.

4. If container is an iframe element, then queue an element task on the DOM manipulation task source given container to run the iframe load event steps given container.

5. Otherwise, if container is non-null, then queue an element task on the DOM manipulation task source given container to fire an event named load at container.

7.10.12 Unloading documents

A Document has a salvageable state, which must initially be true, a fired unload flag, which must initially be false, and a page showing flag, which must initially be false. The page showing flag is used to ensure that scripts receive pageshow and pagehide events in a consistent manner (e.g. that they never receive two pagehide events in a row without an intervening pageshow, or vice versa).

Event loops have a termination nesting level counter, which must initially be 0.

To prompt to unload, given a Document object document and optionally a recursiveFlag, run these steps:

1. Increase the event loop's termination nesting level by 1.

2. Increase the document's ignore-opens-during-unload counter by 1.

3. Let event be the result of creating an event using BeforeUnloadEvent.

4. Initialize event's type attribute to beforeunload and its cancelable attribute true.

5. Dispatch: Dispatch event at document's relevant global object.

6. Decrease the event loop's termination nesting level by 1.

7. If any event listeners were triggered by the earlier dispatch step, then set document's salvageable state to false.

8. If document's active sandboxing flag set does not have its sandboxed modals flag set, and the returnValue attribute of the event object is not the empty string, or if the event was canceled, then the user agent may ask the user to confirm that they wish to unload the document.

   The message shown to the user is not customizable, but instead determined by the user agent. In particular, the actual value of the returnValue attribute is ignored.

   The user agent is encouraged to avoid asking the user for confirmation if it judges that doing so would be annoying, deceptive, or pointless. A simple heuristic might be that if the user has not interacted with the document, the user agent would not ask for confirmation before unloading it.

   If the user agent asks the user for confirmation, it must pause while waiting for the user's response.

   If the user did not confirm the page navigation, then the user agent refused to allow the document to be unloaded.

9. If the recursiveFlag is not set, then:

   1. Let descendants be the list of the descendant browsing contexts of document.

   2. For each browsingContext in descendants:

      1. Prompt to unload browsingContext's active document with the recursiveFlag set. If the user refused to allow the document to be unloaded, then the user implicitly also refused to allow document to be unloaded; break.

      2. If the salvageable state of browsingContext's active document is false, then set the salvageable state of document to false.

10. Decrease the document's ignore-opens-during-unload counter by 1.

To unload a Document document, optionally given a recursiveFlag:

1. Increase the event loop's termination nesting level by one.

2. Increase document's ignore-opens-during-unload counter by one.

3. If document's page showing flag is false, then jump to the step labeled unload event below (i.e. skip firing the pagehide event and don't rerun the unloading document visibility change steps).

4. Set document's page showing flag to false.

5. Run any unloading document visibility change steps for document that are defined by other applicable specifications.

   This is specifically intended for use by Page Visibility. [PAGEVIS]

6. Fire an event named pagehide at document's relevant global object, using PageTransitionEvent, with the persisted attribute initialized to true if document's salvageable state is true, and false otherwise, and legacy target override flag set.

7. Unload event: If document's fired unload flag is false, then fire an event named unload at document's relevant global object, with legacy target override flag set.

8. Decrease the event loop's termination nesting level by one.

9. If any event listeners were triggered by the earlier unload event step, then set document's salvageable state to false and set document's fired unload flag to true.

10. Run any unloading document cleanup steps for document that are defined by this specification and other applicable specifications.

11. If the recursiveFlag is not set, then:

    1. Let descendants be the list of the descendant browsing contexts of document.

    2. For each browsingContext in descendants:

       1. Unload the active document of browsingContext with the recursiveFlag set.

       2. If the salvageable state of the active document of browsingContext is false, then set the salvageable state of document to false also.

    3. If document's salvageable state is false, then discarddocument.

12. Decrease document's ignore-opens-during-unload counter by one.

This specification defines the following unloading document cleanup steps. Other specifications can define more. Given a Document document:

1. Let window be document's relevant global object.

2. For each WebSocket object webSocket whose relevant global object is window, make disappear webSocket.

   If this affected any WebSocket objects, then set document's salvageable state to false.

3. If document's salvageable state is false, then:

   1. For each EventSource object eventSource whose relevant global object is equal to window, forcibly close eventSource.

   2. Empty window's list of active timers.

7.10.12.1 The BeforeUnloadEvent interface

[Exposed=Window]
interface BeforeUnloadEvent : Event {
  attribute DOMString returnValue;
};

There are no BeforeUnloadEvent-specific initialization methods.

The BeforeUnloadEvent interface is a legacy interface which allows prompting to unload to be controlled not only by canceling the event, but by setting the returnValue attribute to a value besides the empty string. Authors should use the preventDefault() method, or other means of canceling events, instead of using returnValue.

The returnValue attribute controls the process of prompting to unload. When the event is created, the attribute must be set to the empty string. On getting, it must return the last value it was set to. On setting, the attribute must be set to the new value.

This attribute is a DOMString only for historical reasons. Any value besides the empty string will be treated as a request to ask the user for confirmation.

7.10.13 Aborting a document load

To abort a Document document:

1. Abort the active documents of every child browsing context. If this results in any of those Document objects having their salvageable state set to false, then set document's salvageable state to false also.

2. Cancel any instances of the fetch algorithm in the context of document, discarding any tasks queued for them, and discarding any further data received from the network for them. If this resulted in any instances of the fetch algorithm being canceled or any queued tasks or any network data getting discarded, then set document's salvageable state to false.

3. If document has an active parser, then:

   1. Set document's active parser was aborted to true.

   2. Abort that parser.

   3. Set document's salvageable state to false.

User agents may allow users to explicitly invoke the abort a document algorithm for a Document. If the user does so, then, if that Document is an active document, the user agent should queue a task to fire an event named abort at that Document object's relevant global object before invoking the abort algorithm.

To stop document loading given a Document object document, run these steps:

1. If document is not an active document, then return.

2. Let browsingContext be document's browsing context.

3. If there is an existing attempt to navigate browsingContext and that attempt is not currently running the unload a document algorithm, then cancel that navigation.

4. Abort document.

7.10.14 The `X-Frame-Options` header

The `X-Frame-Options` HTTP response header is a legacy way of controlling whether and how a Document may be loaded inside of a child browsing context. It is obsoleted by the frame-ancestors CSP directive, which provides more granular control over the same situations. It was originally defined in HTTP Header Field X-Frame-Options, but the definition and processing model here supersedes that document. [CSP] [RFC7034]

In particular, HTTP Header Field X-Frame-Options specified an `ALLOW-FROM` variant of the header, but that is not to be implemented.

Per the below processing model, if both a CSP frame-ancestors directive and an `X-Frame-Options` header are used in the same response, then `X-Frame-Options` is ignored.

For web developers and conformance checkers, its value ABNF is:

X-Frame-Options = "DENY" / "SAMEORIGIN"

To check a navigation response's adherence to `X-Frame-Options`, given a response response, a browsing context browsingContext, and an origin destinationOrigin:

1. If browsingContext is not a child browsing context, then return true.

2. For each policy of response's CSP list:

   1. If policy's disposition is not "enforce", then continue.

   2. If policy's directive set contains a frame-ancestors directive, then return true.

3. Let rawXFrameOptions be the result of getting, decoding, and splitting `X-Frame-Options` from response's header list.

4. Let xFrameOptions be a new set.

5. For each value of rawXFrameOptions, append value, converted to ASCII lowercase, to xFrameOptions.

6. If xFrameOptions's size is greater than 1, and xFrameOptions contains any of "deny", "allowall", or "sameorigin", then return false.

   The intention here is to block any attempts at applying `X-Frame-Options` which were trying to do something valid, but appear confused.

   This is the only impact of the legacy `ALLOWALL` value on the processing model.

7. If xFrameOptions's size is greater than 1, then return true.

   This means it contains multiple invalid values, which we treat the same way as if the header was omitted entirely.

8. If xFrameOptions[0] is "deny", then return false.

9. If xFrameOptions[0] is "sameorigin", then:

   1. Let containerDocument be browsingContext's container document.

   2. While containerDocument is not null:

      1. If containerDocument's origin is not same origin with destinationOrigin, then return false.

      2. Let containerBC be containerDocument's browsing context.

      3. Set containerDocument to containerBC's container document, if containerBC is non-null; otherwise, null.

10. Return true.

    If we've reached this point then we have a lone invalid value (which could potentially be one the legacy `ALLOWALL` or `ALLOW-FROM` forms). These are treated as if the header were omitted entirely.