HTML: The Living Standard

Edition for Web Developers — Last Updated 21 November 2024

Welcome to the dragon's maw. Navigation, session history, and the traversal through that session history are some of the most complex parts of this standard.

The basic concept may not seem so difficult:

The user is looking at a navigable that is presenting its active document. They navigate it to another URL.
The browser fetches the given URL from the network, using it to populate a new session history entry with a newly-created Document.
The browser updates the navigable's active session history entry to the newly-populated one, and thus updates the active document that it is showing to the user.
At some point later, the user presses the browser back button to go back to the previous session history entry.
The browser looks at the URL stored in that session history entry, and uses it to re-fetch and populate that entry's document.
The browser again updates the navigable's active session history entry.

You can see some of the intertwined complexity peeking through here, in how traversal can cause a navigation (i.e., a network fetch to a stored URL), and how a navigation necessarily needs to interface with the session history list to ensure that when it finishes the user is looking at the right thing. But the real problems come in with the various edge cases and interacting web platform features:

Child navigables (e.g., those contained in iframes) can also navigate and traverse, but those navigations need to be linearized into a single session history list since the user only has a single back/forward interface for the entire traversable navigable (e.g., browser tab).
Since the user can traverse back more than a single step in the session history (e.g., by holding down their back button), they can end up traversing multiple navigables at the same time when child navigables are involved. This needs to be synchronized across all of the involved navigables, which might involve multiple event loops or even agent clusters.
During navigation, servers can respond with 204 or 205 status codes or with `Content-Disposition: attachment` headers, which cause navigation to abort and the navigable to stay on its original active document. (This is much worse if it happens during a traversal-initiated navigation!)
Various other HTTP headers, such as `Location`, `Refresh`, `X-Frame-Options`, and those for Content Security Policy, contribute to either the fetching process, or the Document-creation process, or both. The `Cross-Origin-Opener-Policy` header even contributes to the browsing context selection and creation process!
Some navigations (namely fragment navigations and single-page app navigations) are synchronous, meaning that JavaScript code expects to observe the navigation's results instantly. This then needs to be synchronized with the view of the session history that all other navigables in the tree see, which can be subject to race conditions and necessitate resolving conflicting views of the session history.
The platform has accumulated various exciting navigation-related features that need special-casing, such as javascript: URLs, srcdoc iframes, and the beforeunload event.

In what follows, we have attempted to guide the reader through these complexities by appropriately cordoning them off into labeled sections and algorithms, and giving appropriate words of introduction where possible. Nevertheless, if you wish to truly understand navigation and session history, the usual advice will be invaluable.

7.4.1 Session history

7.4.1.1 Session history entries

A session history entry is a struct with the following items:

step, a non-negative integer or "pending", initially "pending".
URL, a URL
document state, a document state.
classic history API state, which is serialized state, initially StructuredSerializeForStorage(null).
navigation API state, which is a serialized state, initially StructuredSerializeForStorage(undefined).
navigation API key, which is a string, initially set to the result of generating a random UUID.
navigation API ID, which is a string, initially set to the result of generating a random UUID.
scroll restoration mode, a scroll restoration mode, initially "auto".
scroll position data, which is scroll position data for the document's restorable scrollable regions.
persisted user state, which is implementation-defined, initially null

For example, some user agents might want to persist the values of form controls.

User agents that persist the value of form controls are encouraged to also persist their directionality (the value of the element's dir attribute). This prevents values from being displayed incorrectly after a history traversal when the user had originally entered the values with an explicit, non-default directionality.

To get a session history entry's document, return its document state's document.

Serialized state is a serialization (via StructuredSerializeForStorage) of an object representing a user interface state. We sometimes informally refer to "state objects", which are the objects representing user interface state supplied by the author, or alternately the objects created by deserializing (via StructuredDeserialize) serialized state.

Pages can add serialized state to the session history. These are then deserialized and returned to the script when the user (or script) goes back in the history, thus enabling authors to use the "navigation" metaphor even in one-page applications.

Serialized state is intended to be used for two main purposes: first, storing a preparsed description of the state in the URL so that in the simple case an author doesn't have to do the parsing (though one would still need the parsing for handling URLs passed around by users, so it's only a minor optimization). Second, so that the author can store state that one wouldn't store in the URL because it only applies to the current Document instance and it would have to be reconstructed if a new Document were opened.

An example of the latter would be something like keeping track of the precise coordinate from which a popup div was made to animate, so that if the user goes back, it can be made to animate to the same location. Or alternatively, it could be used to keep a pointer into a cache of data that would be fetched from the server based on the information in the URL, so that when going back and forward, the information doesn't have to be fetched again.

A scroll restoration mode indicates whether the user agent should restore the persisted scroll position (if any) when traversing to an entry. A scroll restoration mode is one of the following:

"auto": The user agent is responsible for restoring the scroll position upon navigation.
"manual": The page is responsible for restoring the scroll position and the user agent does not attempt to do so automatically

7.4.1.2 Document state

Document state holds state inside a session history entry regarding how to present and, if necessary, recreate, a Document. It has:

A document, a Document or null, initially null.

When a history entry is active, it has a Document in its document state. However, when a Document is not fully active, it's possible for it to be destroyed to free resources. In such cases, this document item will be nulled out. The URL and other data in the session history entry and document state is then used to bring a new Document into being to take the place of the original, in the case where the user agent finds itself having to traverse to the entry.

If the Document is not destroyed, then during history traversal, it can be reactivated. The cache in which browsers store such Documents is often called a back-forward cache, or bfcache (or perhaps "blazingly fast" cache).
A history policy container, a policy container or null, initially null.
A request referrer, which is "no-referrer", "client", or a URL, initially "client".
A request referrer policy, which is a referrer policy, initially the default referrer policy.

The request referrer policy is distinct from the history policy container's referrer policy. The former is used for fetches of this document, whereas the latter controls fetches by this document.
An initiator origin, which is an origin or null, initially null.
An origin, which is an origin or null, initially null.

This is the origin that we set "about:"-schemed Documents' origin to. We store it here because it is also used when restoring these Documents during traversal, since they are reconstructed locally without visiting the network. It is also used to compare the origin before and after the session history entry is repopulated. If the origins change, the navigable target name is cleared.
An about base URL, which is a URL or null, initially null.

This will be populated only for "about:"-schemed Documents and will be the fallback base URL for those Documents. It is a snapshot of the initiator Document's document base URL.
Nested histories, a list of nested histories, initially an empty list.
A resource, a string, POST resource or null, initially null.

A string is treated as HTML. It's used to store the source of an iframe srcdoc document.
A reload pending boolean, initially false.
An ever populated boolean, initially false.
A navigable target name string, initially the empty string.
A not restored reasons, a not restored reasons or null, initially null.

User agents may destroy a document and its descendants given the documents of document states with non-null documents, as long as the Document is not fully active.

Apart from that restriction, this standard does not specify when user agents should destroy the document stored in a document state, versus keeping it cached.

A POST resource has:

A request body, a byte sequence or failure.

This is only ever accessed in parallel, so it doesn't need to be stored in memory. However, it must return the same byte sequence each time. If this isn't possible due to resources changing on disk, or if resources can no longer be accessed, then this must be set to failure.
A request content-type, which is `application/x-www-form-urlencoded`, `multipart/form-data`, or `text/plain`.

A nested history has:

An id, a unique internal value.

This is used to associate the nested history with a navigable.
Entries, a list of session history entries.

This will later contain ways to identify a child navigable across reloads.

Several contiguous entries in a session history can share the same document state. This can occur when the initial entry is reached via normal navigation, and the following entry is added via history.pushState(). Or it can occur via navigation to a fragment.

All entries that share the same document state (and that are therefore merely different states of one particular document) are contiguous by construction.

A Document has a latest entry, a session history entry or null.

This is the entry that was most recently represented by a given Document. A single Document can represent many session history entries over time, as many contiguous session history entries can share the same document state as explained above.

7.4.1.3 Centralized modifications of session history

To maintain a single source of truth, all modifications to a traversable navigable's session history entries need to be synchronized. This is especially important due to how session history is influenced by all of the descendant navigables, and thus by multiple event loops. To accomplish this, we use the session history traversal parallel queue structure.

A session history traversal parallel queue is very similar to a parallel queue. It has an algorithm set, an ordered set.

The items in a session history traversal parallel queue's algorithm set are either algorithm steps, or synchronous navigation steps, which are a particular brand of algorithm steps involving a target navigable (a navigable).

To append session history traversal steps to a traversable navigable traversable given algorithm steps steps, append steps to traversable's session history traversal queue's algorithm set.

To append session history synchronous navigation steps involving a navigable targetNavigable to a traversable navigable traversable given algorithm steps steps, append steps as synchronous navigation steps targeting target navigable targetNavigable to traversable's session history traversal queue's algorithm set.

To start a new session history traversal parallel queue:

Let sessionHistoryTraversalQueue be a new session history traversal parallel queue.
Run the following steps in parallel:
1. While true:
  1. If sessionHistoryTraversalQueue's algorithm set is empty, then continue.
  2. Let steps be the result of dequeuing from sessionHistoryTraversalQueue's algorithm set.
  3. Run steps.
Return sessionHistoryTraversalQueue.

Imagine the joint session history depicted by this Jake diagram:

	0	1
`top`	/a	/b

And the following code runs at the top level:

history.back();
location.href = '#foo';

The desired result is:

	0	1	2
`top`	/a	/b	/b#foo

This isn't straightforward, as the sync navigation wins the race in terms of being observable, whereas the traversal wins the race in terms of queuing steps on the session history traversal parallel queue. To achieve this result, the following happens:

history.back() appends steps intended to traverse by a delta of −1.
location.href = '#foo' synchronously changes the active session history entry entry to a newly-created one, with the URL /b#foo, and appends synchronous steps to notify the central source of truth about that new entry. Note that this does not yet update the current session history entry, current session history step, or the session history entries list; those updates cannot be done synchronously, and instead must be done as part of the queued steps.
On the session history traversal parallel queue, the steps queued by history.back() run:
1. The target history step is determined to be 0: the current session history step (i.e., 1) plus the intended delta of −1.
2. We enter the main apply the history step algorithm.
  
  The entry at step 0, for the /a URL, has its document populated.
  
  Meanwhile, the queue is checked for synchronous navigation steps. The steps queued by the location.href setter now run, and block the traversal from performing effects beyond document population (such as, unloading documents and switching active history entries) until they are finished. Those steps cause the following to happen:
  1. The entry with URL /b#foo is added, with its step determined to be 2: the current session history step (i.e., 1) plus 1.
  2. We fully switch to that newly added entry, including a nested call to apply the history step. This ultimately results in updating the document by dispatching events like hashchange.
  Only once that is all complete, and the /a history entry has been fully populated with a document, do we move on with applying the history step given the target step of 0.
  
  At this point, the Document with URL /b#foo unloads, and we finish moving to our target history step 0, which makes the entry with URL /a become the active session history entry and 0 become the current session history step.

Here is another more complex example, involving races between populating two different iframes, and a synchronous navigation once one of those iframes loads. We start with this setup:

	0	1	2
`top`	/t
`frames[0]`	/i-0-a	/i-0-b
`frames[1]`	/i-1-a		/i-1-b

and then call history.go(-2). The following then occurs:

history.go(-2) appends steps intended to traverse by a delta of −2. Once those steps run:
1. The target step is determined to be 2 + (−2) = 0.
2. In parallel, the fetches are made to populate the two iframes, fetching /i-0-a and /i-1-a respectively.
  
  Meanwhile, the queue is checked for synchronous navigation steps. There aren't any right now.
3. In the fetch race, the fetch for /i-0-a wins. We proceed onward to finish all of apply the history step's work for how the traversal impacts the frames[0] navigable, including updating its active session history entry to the entry with URL /i-0-a.
4. Before the fetch for /i-1-a finishes, we reach the point where scripts may run for the newly-created document in the frames[0] navigable's active document. Some such script does run:
```
location.href = '#foo'
```
  This synchronously changes the frames[0] navigable's active session history entry entry to a newly-created one, with the URL /i-0-a#foo, and appends synchronous steps to notify the central source of truth about that new entry.
  
  Unlike in the previous example, these synchronous steps do not "jump the queue" and update the traversable before we finish the fetch for /i-1-a. This is because the navigable in question, frames[0], has already been altered as part of the traversal, so we know that with the current session history step being 2, adding the new entry as a step 3 doesn't make sense.
5. Once the fetch for /i-1-a finally finishes, we proceed to finish updating the frames[1] navigable for the traversal, including updating its active session history entry to the entry with URL /i-1-a.
6. Now that both navigables have finished processing the traversal, we update the current session history step to the target step of 0.
Now we can process the steps that were queued for the synchronous navigation:
1. The /i-0-a#foo entry is added, with its step determined to be 1: the current session history step (i.e., 0) plus 1. This also clears existing forward history.
2. We fully switch to that newly added entry, including calling apply the history step. This ultimately results in updating the document by dispatching events like hashchange, as well as updating the current session history step to the target step of 1.

The end result is:

	0	1
`top`	/t
`frames[0]`	/i-0-a	/i-0-a#foo
`frames[1]`	/i-1-a

7.4.1.4 Low-level operations on session history

This section contains a miscellaneous grab-bag of operations that we perform throughout the standard when manipulating session history. The best way to get a sense of what they do is to look at their call sites.

To get session history entries of a navigable navigable:

Let traversable be navigable's traversable navigable.
Assert: this is running within traversable's session history traversal queue.
If navigable is traversable, return traversable's session history entries.
Let docStates be an empty ordered set of document states.
For each entry of traversable's session history entries, append entry's document state to docStates.
For each docState of docStates:
1. For each nestedHistory of docState's nested histories:
  1. If nestedHistory's id equals navigable's id, return nestedHistory's entries.
  2. For each entry of nestedHistory's entries, append entry's document state to docStates.
Assert: this step is not reached.

To get session history entries for the navigation API of a navigable navigable given an integer targetStep:

Let rawEntries be the result of getting session history entries for navigable.
Let entriesForNavigationAPI be a new empty list.
Let startingIndex be the index of the session history entry in rawEntries who has the greatest step less than or equal to targetStep.

See this example to understand why it's the greatest step less than or equal to targetStep.
Append rawEntries[startingIndex] to entriesForNavigationAPI.
Let startingOrigin be rawEntries[startingIndex]'s document state's origin.
Let i be startingIndex − 1.
While i > 0:
1. If rawEntries[i]'s document state's origin is not same origin with startingOrigin, then break.
2. Prepend rawEntries[i] to entriesForNavigationAPI.
3. Set i to i − 1.
Set i to startingIndex + 1.
While i < rawEntries's size:
1. If rawEntries[i]'s document state's origin is not same origin with startingOrigin, then break.
2. Append rawEntries[i] to entriesForNavigationAPI.
3. Set i to i + 1.
Return entriesForNavigationAPI.

To clear the forward session history of a traversable navigable navigable:

Assert: this is running within navigable's session history traversal queue.
Let step be the navigable's current session history step.
Let entryLists be the ordered set « navigable's session history entries ».
For each entryList of entryLists:
1. Remove every session history entry from entryList that has a step greater than step.
2. For each entry of entryList:
  1. For each nestedHistory of entry's document state's nested histories, append nestedHistory's entries list to entryLists.

To get all used history steps that are part of traversable navigable traversable:

Assert: this is running within traversable's session history traversal queue.
Let steps be an empty ordered set of non-negative integers.
Let entryLists be the ordered set « traversable's session history entries ».
For each entryList of entryLists:
1. For each entry of entryList:
  1. Append entry's step to steps.
  2. For each nestedHistory of entry's document state's nested histories, append nestedHistory's entries list to entryLists.
Return steps, sorted.

7.4.2 Navigation

Certain actions cause a navigable to navigate to a new resource.

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

Although in this standard the word "navigation" refers specifically to the navigate algorithm, this doesn't always line up with web developer or user perceptions. For example:

The URL and history update steps are often used during so-called "single-page app navigations" or "same-document navigations", but they do not trigger the navigate algorithm.
Reloads and traversals are sometimes talked about as a type of navigation, since all three will often attempt to populate the history entry's document and thus could perform navigational fetches. See, e.g., the APIs exposed Navigation Timing. But they have their own entry point algorithms, separate from the navigate algorithm. [NAVIGATIONTIMING]
Although fragment navigations are always done through the navigate algorithm, a user might perceive them as more like jumping around a single page, than as a true navigation.

Before we can jump into the navigation algorithm itself, we need to establish several important structures that it uses.

The source snapshot params struct is used to capture data from a Document initiating a navigation. It is snapshotted at the beginning of a navigation and used throughout the navigation's lifetime. It has the following items:

has transient activation: a boolean
sandboxing flags: a sandboxing flag set
allows downloading: a boolean
fetch client: an environment settings object, only to be used as a request client
source policy container: a policy container

To snapshot source snapshot params given a Document sourceDocument, return a new source snapshot params with

has transient activation: true if sourceDocument's relevant global object has transient activation; otherwise false
sandboxing flags: sourceDocument's active sandboxing flag set
allows downloading: false if sourceDocument's active sandboxing flag set has the sandboxed downloads browsing context flag set; otherwise true
fetch client: sourceDocument's relevant settings object
source policy container: sourceDocument's policy container

The target snapshot params struct is used to capture data from a navigable being navigated. Like source snapshot params, it is snapshotted at the beginning of a navigation and used throughout the navigation's lifetime. It has the following items:

sandboxing flags: a sandboxing flag set

To snapshot target snapshot params given a navigable targetNavigable, return a new target snapshot params with sandboxing flags set to the result of determining the creation sandboxing flags given targetNavigable's active browsing context and targetNavigable's container.

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 that algorithm are tracked via a navigation params struct, which has the following items:

id: null or a navigation ID
navigable: the navigable to be navigated
request: null or a request that started the navigation
response: a response that ultimately was navigated to (potentially a network error)
fetch controller: null or a fetch controller
commit early hints: null or an algorithm accepting a Document, once it has been created
COOP enforcement result: an opener policy enforcement result, used for reporting and potentially for causing a browsing context group switch
reserved environment: null or an environment reserved for the new Document
origin: an origin to use for the new Document
policy container: a policy container to use for the new Document
final sandboxing flag set: a sandboxing flag set to impose on the new Document
opener policy: an opener policy to use for the new Document
navigation timing type: a NavigationTimingType used for creating the navigation timing entry for the new Document
about base URL: a URL or null used to populate the new Document's about base URL

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

A navigation ID is a UUID string generated during navigation. It is used to interface with the WebDriver BiDi specification as well as to track the ongoing navigation. [WEBDRIVERBIDI]

After Document creation, the relevant traversable navigable's session history gets updated. The NavigationHistoryBehavior enumeration is used to indicate the desired type of session history update to the navigate algorithm. It is one of the following:

"push": A regular navigation which adds a new session history entry, and will clear the forward session history.
"replace": A navigation that will replace the active session history entry.
"auto": The default value, which will be converted very early in the navigate algorithm into "push" or "replace". Usually it becomes "push", but under certain circumstances it becomes "replace" instead.

A history handling behavior is a NavigationHistoryBehavior that is either "push" or "replace", i.e., that has been resolved away from any initial "auto" value.

The navigation must be a replace, given a URL url and a Document document, if any of the following are true:

url's scheme is "javascript"; or
document's is initial about:blank is true.

Other cases that often, but not always, force a "replace" navigation are:

if the Document is not completely loaded; or
if the target URL equals the Document's URL.

Various parts of the platform track whether a user is involved in a navigation. A user navigation involvement is one of the following:

"browser UI": The navigation was initiated by the user via browser UI mechanisms.
"activation": The navigation was initiated by the user via the activation behavior of an element.
"none": The navigation was not initiated by the user.

For convenience at certain call sites, the user navigation involvement for an Event event is defined as follows:

Assert: this algorithm is being called as part of an activation behavior definition.
Assert: event's type is "click".
If event's isTrusted is initialized to true, then return "activation".
Return "none".

To navigate a navigable navigable to a URL url using a Document sourceDocument, with an optional POST resource, string, or null documentResource (default null), an optional response-or-null response (default null), an optional boolean exceptionsEnabled (default false), an optional NavigationHistoryBehavior historyHandling (default "auto"), an optional serialized state-or-null navigationAPIState (default null), an optional entry list or null formDataEntryList (default null), an optional referrer policy referrerPolicy (default the empty string), and an optional user navigation involvement userInvolvement (default "none"):

Let cspNavigationType be "form-submission" if formDataEntryList is non-null; otherwise "other".
Let sourceSnapshotParams be the result of snapshotting source snapshot params given sourceDocument.
Let initiatorOriginSnapshot be sourceDocument's origin.
Let initiatorBaseURLSnapshot be sourceDocument's document base URL.
If sourceDocument's node navigable is not allowed by sandboxing to navigate navigable given sourceSnapshotParams, then:
1. If exceptionsEnabled is true, then throw a "SecurityError" DOMException.
2. Return.
Let navigationId be the result of generating a random UUID. [WEBCRYPTO]
If the surrounding agent is equal to navigable's active document's relevant agent, then continue these steps. Otherwise, queue a global task on the navigation and traversal task source given navigable's active window to continue these steps.

We do this because we are about to look at a lot of properties of navigable's active document, which are in theory only accessible over in the appropriate event loop. (But, we do not want to unconditionally queue a task, since — for example — same-event-loop fragment navigations need to take effect synchronously.)

Another implementation strategy would be to replicate the relevant information across event loops, or into a canonical "browser process", so that it can be consulted without queueing a task. This could give different results than what we specify here in edge cases, where the relevant properties have changed over in the target event loop but not yet been replicated. Further testing is needed to determine which of these strategies best matches browser behavior, in such racy edge cases.
If navigable's active document's unload counter is greater than 0, then invoke WebDriver BiDi navigation failed with a WebDriver BiDi navigation status whose id is navigationId, status is "canceled", and url is url, and return.
Let container be navigable's container.
If container is an iframe element and will lazy load element steps given container returns true, then stop intersection-observing a lazy loading element container and set container's lazy load resumption steps to null.
If historyHandling is "auto", then:
1. If url equals navigable's active document's URL, and initiatorOriginSnapshot is same origin with targetNavigable's active document's origin, then set historyHandling to "replace".
2. Otherwise, set historyHandling to "push".
If the navigation must be a replace given url and navigable's active document, then set historyHandling to "replace".
If all of the following are true:
- documentResource is null;
- response is null;
- url equals navigable's active session history entry's URL with exclude fragments set to true; and
- url's fragment is non-null,
then:
1. Navigate to a fragment given navigable, url, historyHandling, userInvolvement, navigationAPIState, and navigationId.
2. Return.
If navigable's parent is non-null, then set navigable's is delaying load events to true.
Let targetBrowsingContext be navigable's active browsing context.
Let targetSnapshotParams be the result of snapshotting target snapshot params given navigable.
Invoke WebDriver BiDi navigation started with targetBrowsingContext, and a new WebDriver BiDi navigation status whose id is navigationId, status is "pending", and url is url.
If navigable's ongoing navigation is "traversal", then:
1. Invoke WebDriver BiDi navigation failed with targetBrowsingContext and a new WebDriver BiDi navigation status whose id is navigationId, status is "canceled", and url is url.
2. Return.
Any attempts to navigate a navigable that is currently traversing are ignored.
Set the ongoing navigation for navigable to navigationId.

This will have the effect of aborting other ongoing navigations of navigable, since at certain points during navigation changes to the ongoing navigation will cause further work to be abandoned.
If url's scheme is "javascript", then:
1. Queue a global task on the navigation and traversal task source given navigable's active window to navigate to a javascript: URL given navigable, url, historyHandling, initiatorOriginSnapshot, and cspNavigationType.
2. Return.
If all of the following are true:
- userInvolvement is not "browser UI";
- navigable's active document's origin is same origin-domain with sourceDocument's origin;
- navigable's active document's is initial about:blank is false; and
- url's scheme is a fetch scheme,
then:
1. Let navigation be navigable's active window's navigation API.
2. Let entryListForFiring be formDataEntryList if documentResource is a POST resource; otherwise, null.
3. Let navigationAPIStateForFiring be navigationAPIState if navigationAPIState is not null; otherwise, StructuredSerializeForStorage(undefined).
4. Let continue be the result of firing a push/replace/reload navigate event at navigation with navigationType set to historyHandling, isSameDocument set to false, userInvolvement set to userInvolvement, formDataEntryList set to entryListForFiring, destinationURL set to url, and navigationAPIState set to navigationAPIStateForFiring.
5. If continue is false, then return.
It is possible for navigations with userInvolvement of "browser UI" or initiated by a cross origin-domain sourceDocument to fire navigate events, if they go through the earlier navigate to a fragment path.
In parallel, run these steps:
1. Let unloadPromptCanceled be the result of checking if unloading is canceled for navigable's active document's inclusive descendant navigables.
2. If unloadPromptCanceled is true, or navigable's ongoing navigation is no longer navigationId, then:
  1. Invoke WebDriver BiDi navigation failed with targetBrowsingContext and a new WebDriver BiDi navigation status whose id is navigationId, status is "canceled", and url is url.
  2. Abort these steps.
3. Queue a global task on the navigation and traversal task source given navigable's active window to abort a document and its descendants given navigable's active document.
4. Let documentState be a new document state with
  
  request referrer policy
  referrerPolicy
  initiator origin
  initiatorOriginSnapshot
  resource
  documentResource
  navigable target name
  navigable's target name
  
  The navigable target name can get cleared under various conditions later in the navigation process, before the document state is finalized.
5. If url matches about:blank or is about:srcdoc, then:
  1. Set documentState's origin to initiatorOriginSnapshot.
  2. Set documentState's about base URL to initiatorBaseURLSnapshot.
6. Let historyEntry be a new session history entry, with its URL set to url and its document state set to documentState.
7. Let navigationParams be null.
8. If response is non-null:
  
  The navigate algorithm is only supplied with a response as part of the object and embed processing models, or for processing parts of multipart/x-mixed-replace responses after the initial response.
  1. Let policyContainer be the result of determining navigation params policy container given response's URL, null, a clone of the sourceDocument's policy container, navigable's container document's policy container, and null.
  2. Let finalSandboxFlags be the union of targetSnapshotParams's sandboxing flags and policyContainer's CSP list's CSP-derived sandboxing flags.
  3. Let responseOrigin be the result of determining the origin given response's URL, finalSandboxFlags, and documentState's initiator origin.
  4. Let coop be a new opener policy.
  5. Let coopEnforcementResult be a new opener policy enforcement result with
    
    url
    response's URL
    origin
    responseOrigin
    opener policy
    coop
  6. Set navigationParams to a new navigation params, with
    
    id
    navigationId
    navigable
    navigable
    request
    null
    response
    response
    fetch controller
    null
    commit early hints
    null
    COOP enforcement result
    coopEnforcementResult
    reserved environment
    null
    origin
    responseOrigin
    policy container
    policyContainer
    final sandboxing flag set
    finalSandboxFlags
    opener policy
    coop
    navigation timing type
    "navigate"
    about base URL
    documentState's about base URL
9. Attempt to populate the history entry's document for historyEntry, given navigable, "navigate", sourceSnapshotParams, targetSnapshotParams, navigationId, navigationParams, cspNavigationType, with allowPOST set to true and completionSteps set to the following step:
  1. Append session history traversal steps to navigable's traversable to finalize a cross-document navigation given navigable, historyHandling, and historyEntry.

Although the usual cross-document navigation case will first foray into populating a session history entry with a Document, all navigations that don't get aborted will ultimately end up calling into one of the below algorithms.

To finalize a cross-document navigation given a navigable navigable, history handling behavior historyHandling, and session history entry historyEntry:

Assert: this is running on navigable's traversable navigable's session history traversal queue.
Set navigable's is delaying load events to false.
If historyEntry's document is null, then return.

This means that attempting to populate the history entry's document ended up not creating a document, as a result of e.g., the navigation being canceled by a subsequent navigation, a 204 No Content response, etc.
If all of the following are true:
- navigable's parent is null;
- historyEntry's document's browsing context is not an auxiliary browsing context whose opener browsing context is non-null; and
- historyEntry's document's origin is not navigable's active document's origin,
then set historyEntry's document state's navigable target name to the empty string.
Let entryToReplace be navigable's active session history entry if historyHandling is "replace", otherwise null.
Let traversable be navigable's traversable navigable.
Let targetStep be null.
Let targetEntries be the result of getting session history entries for navigable.
If entryToReplace is null, then:
1. Clear the forward session history of traversable.
2. Set targetStep to traversable's current session history step + 1.
3. Set historyEntry's step to targetStep.
4. Append historyEntry to targetEntries.
Otherwise:
1. Replace entryToReplace with historyEntry in targetEntries.
2. Set historyEntry's step to entryToReplace's step.
3. If historyEntry's document state's origin is same origin with entryToReplace's document state's origin, then set historyEntry's navigation API key to entryToReplace's navigation API key.
4. Set targetStep to traversable's current session history step.
Apply the push/replace history step targetStep to traversable given historyHandling.

7.4.2.3.2 The `javascript:` URL special case

javascript: URLs have a dedicated label on the issue tracker documenting various problems with their specification.

To navigate to a javascript: URL, given a navigable targetNavigable, a URL url, a history handling behavior historyHandling, an origin initiatorOrigin, and a string cspNavigationType:

Assert: historyHandling is "replace".
Set the ongoing navigation for targetNavigable to null.
If initiatorOrigin is not same origin-domain with targetNavigable's active document's origin, then return.
Let request be a new request whose URL is url.

This is a synthetic request solely for plumbing into the next step. It will never hit the network.
If the result of should navigation request of type be blocked by Content Security Policy? given request and cspNavigationType is "Blocked", then return. [CSP]
Let newDocument be the result of evaluating a javascript: URL given targetNavigable, url, and initiatorOrigin.
If newDocument is null, then return.

In this case, some JavaScript code was executed, but no new Document was created, so we will not perform a navigation.
Assert: initiatorOrigin is newDocument's origin.
Let entryToReplace be targetNavigable's active session history entry.
Let oldDocState be entryToReplace's document state.
Let documentState be a new document state with

document
newDocument
history policy container
a clone of the oldDocState's history policy container if it is non-null; null otherwise
request referrer
oldDocState's request referrer
request referrer policy
oldDocState's request referrer policy or should this be the referrerPolicy that was passed to navigate?
initiator origin
initiatorOrigin
origin
initiatorOrigin
about base URL
oldDocState's about base URL
resource
null
ever populated
true
navigable target name
oldDocState's navigable target name
Let historyEntry be a new session history entry, with

URL
entryToReplace's URL
document state
documentState

For the URL, we do not use url, i.e. the actual javascript: URL that the navigate algorithm was called with. This means javascript: URLs are never stored in session history, and so can never be traversed to.
Append session history traversal steps to targetNavigable's traversable to finalize a cross-document navigation with targetNavigable, historyHandling, and historyEntry.

To evaluate a javascript: URL given a navigable targetNavigable, a URL url, and an origin newDocumentOrigin:

Let urlString be the result of running the URL serializer on url.
Let encodedScriptSource be the result of removing the leading "javascript:" from urlString.
Let scriptSource be the UTF-8 decoding of the percent-decoding of encodedScriptSource.
Let settings be targetNavigable's active document's relevant settings object.
Let baseURL be settings's API base URL.
Let script be the result of creating a classic script given scriptSource, settings, baseURL, and the default script fetch options.
Let evaluationStatus be the result of running the classic script script.
Let result be null.
If evaluationStatus is a normal completion, and evaluationStatus.[[Value]] is a String, then set result to evaluationStatus.[[Value]].
Otherwise, return null.
Let response be a new response with

URL
targetNavigable's active document's URL
header list
« (`Content-Type`, `text/html;charset=utf-8`) »
body
the UTF-8 encoding of result, as a body

The encoding to UTF-8 means that unpaired surrogates will not roundtrip, once the HTML parser decodes the response body.
Let policyContainer be targetNavigable's active document's policy container.
Let finalSandboxFlags be policyContainer's CSP list's CSP-derived sandboxing flags.
Let coop be targetNavigable's active document's opener policy.
Let coopEnforcementResult be a new opener policy enforcement result with

url
url
origin
newDocumentOrigin
opener policy
coop
Let navigationParams be a new navigation params, with

id
navigationId
navigable
targetNavigable
request
null this will cause the referrer of the resulting Document to be null; is that correct?
response
response
fetch controller
null
commit early hints
null
COOP enforcement result
coopEnforcementResult
reserved environment
null
origin
newDocumentOrigin
policy container
policyContainer
final sandboxing flag set
finalSandboxFlags
opener policy
coop
navigation timing type
"navigate"
about base URL
targetNavigable's active document's about base URL
Return the result of loading an HTML document given navigationParams.

7.4.2.3.3 Fragment navigations

To navigate to a fragment given a navigable navigable, a URL url, a history handling behavior historyHandling, a user navigation involvement userInvolvement, a serialized state-or-null navigationAPIState, and a navigation ID navigationId:

Let navigation be navigable's active window's navigation API.
Let destinationNavigationAPIState be navigable's active session history entry's navigation API state.
If navigationAPIState is not null, then set destinationNavigationAPIState to navigationAPIState.
Let continue be the result of firing a push/replace/reload navigate event at navigation with navigationType set to historyHandling, isSameDocument set to true, userInvolvement set to userInvolvement, destinationURL set to url, and navigationAPIState set to destinationNavigationAPIState.
If continue is false, then return.
Let historyEntry be a new session history entry, with

URL
url
document state
navigable's active session history entry's document state
navigation API state
destinationNavigationAPIState
scroll restoration mode
navigable's active session history entry's scroll restoration mode

For navigations peformed with navigation.navigate(), the value provided by the state option is used for the new navigation API state. (This will set it to the serialization of undefined, if no value is provided for that option.) For other fragment navigations, including user-initiated ones, the navigation API state is carried over from the previous entry.

The classic history API state is never carried over.
Let entryToReplace be navigable's active session history entry if historyHandling is "replace", otherwise null.
Let history be navigable's active document's history object.
Let scriptHistoryIndex be history's index.
Let scriptHistoryLength be history's length.
If historyHandling is "push", then:
1. Set history's state to null.
2. Increment scriptHistoryIndex.
3. Set scriptHistoryLength to scriptHistoryIndex + 1.
Set navigable's active document's URL to url.
Set navigable's active session history entry to historyEntry.
Update document for history step application given navigable's active document, historyEntry, true, scriptHistoryIndex, scriptHistoryLength, and historyHandling.

This algorithm will be called twice as a result of a single fragment navigation: once synchronously, where best-guess values scriptHistoryIndex and scriptHistoryLength are set, history.state is nulled out, and various events are fired; and once asynchronously, where the final values for index and length are set, history.state remains untouched, and no events are fired.
Scroll to the fragment given navigable's active document.

If the scrolling fails because the Document is new and the relevant ID has not yet been parsed, then the second asynchronous call to update document for history step application will take care of scrolling.
Let traversable be navigable's traversable navigable.
Append the following session history synchronous navigation steps involving navigable to traversable:
1. Finalize a same-document navigation given traversable, navigable, historyEntry, entryToReplace, and historyHandling.
2. Invoke WebDriver BiDi fragment navigated with navigable's active browsing context and a new WebDriver BiDi navigation status whose id is navigationId, url is url, and status is "complete".

To finalize a same-document navigation given a traversable navigable traversable, a navigable targetNavigable, a session history entry targetEntry, a session history entry-or-null entryToReplace, and a history handling behavior historyHandling:

This is used by both fragment navigations and by the URL and history update steps, which are the only synchronous updates to session history. By virtue of being synchronous, those algorithms are performed outside of the top-level traversable's session history traversal queue. This puts them out of sync with the top-level traversable's current session history step, so this algorithm is used to resolve conflicts due to race conditions.

Assert: this is running on traversable's session history traversal queue.
If targetNavigable's active session history entry is not targetEntry, then return.
Let targetStep be null.
Let targetEntries be the result of getting session history entries for targetNavigable.
If entryToReplace is null, then:
1. Clear the forward session history of traversable.
2. Set targetStep to traversable's current session history step + 1.
3. Set targetEntry's step to targetStep.
4. Append targetEntry to targetEntries.
Otherwise:
1. Replace entryToReplace with targetEntry in targetEntries.
2. Set targetEntry's step to entryToReplace's step.
3. Set targetStep to traversable's current session history step.
Apply the push/replace history step targetStep to traversable given historyHandling.

This is done even for "replace" navigations, as it resolves race conditions across multiple synchronous navigations.

7.4.2.3.4 Non-fetch schemes and external software

The input to attempt to create a non-fetch scheme document is the non-fetch scheme navigation params struct. It is a lightweight version of navigation params which only carries parameters relevant to the non-fetch scheme navigation case. It has the following items:

id

null or a

navigable

the navigable experiencing the navigation

URL

a URL

target snapshot sandboxing flags

the target snapshot params's sandboxing flags present during navigation

source snapshot has transient activation

a copy of the source snapshot params's has transient activation boolean present during activation

initiator origin

an origin possibly for use in a user-facing prompt to confirm the invocation of an external software package

This differs slightly from a document state's initiator origin in that a non-fetch scheme navigation params's initiator origin follows redirects up to the last fetch scheme URL in a redirect chain that ends in a non-fetch scheme URL.

navigation timing type

a NavigationTimingType used for for the new Document

To attempt to create a non-fetch scheme document, given a non-fetch scheme navigation params navigationParams:

Let url be navigationParams's URL.
Let navigable be navigationParams's navigable.
If url is to be handled using a mechanism that does not affect navigable, e.g., because url's scheme is handled externally, then:
1. Hand-off to external software given url, navigable, navigationParams's target snapshot sandboxing flags, navigationParams's source snapshot has transient activation, and navigationParams's initiator origin.
2. Return null.
Handle url 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. Return the result of displaying the inline content given navigable, navigationParams's id, and navigationParams's navigation timing type.

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

To hand-off to external software given a URL or response resource, a navigable navigable, a sandboxing flag set sandboxFlags, a boolean hasTransientActivation, and an origin initiatorOrigin user agents should:

If all of the following are true:
- navigable is not a top-level traversable;
- sandboxFlags has its sandboxed custom protocols navigation browsing context flag set; and
- sandboxFlags has its sandboxed top-level navigation with user activation browsing context flag set, or hasTransientActivation is false,
then return without invoking the external software package.

Navigation inside an iframe toward external software can be seen by users as a new popup or a new top-level navigation. That's why its is allowed in sandboxed iframe only when one of allow-popups, allow-top-navigation, allow-top-navigation-by-user-activation, or allow-top-navigation-to-custom-protocols is specified.
Perform the appropriate handoff of resource while attempting to mitigate the risk that this is an attempt to exploit the target software. For example, user agents could prompt the user to confirm that initiatorOrigin is to be allowed to invoke the external software in question. In particular, if hasTransientActivation is false, then 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.

A couple of scenarios can intervene early in the navigation process and put the whole thing to a halt. This can be especially exciting when multiple navigables are navigating at the same time, due to a session history traversal.

A navigable source is allowed by sandboxing to navigate a second navigable target, given a source snapshot params sourceSnapshotParams, if the following steps return true:

If source is target, then return true.
If source is an ancestor of target, then return true.
If target is an ancestor of source, then:
1. If target is not a top-level traversable, then return true.
2. If sourceSnapshotParams's has transient activation is true, and sourceSnapshotParams's sandboxing flags's sandboxed top-level navigation with user activation browsing context flag is set, then return false.
3. If sourceSnapshotParams's has transient activation is false, and sourceSnapshotParams's sandboxing flags's sandboxed top-level navigation without user activation browsing context flag is set, then return false.
4. Return true.
If target is a top-level traversable:
1. If source is the one permitted sandboxed navigator of target, then return true.
2. If sourceSnapshotParams's sandboxing flags's sandboxed navigation browsing context flag is set, then return false.
3. Return true.
If sourceSnapshotParams's sandboxing flags's sandboxed navigation browsing context flag is set, then return false.
Return true.

To check if unloading is canceled for a list of navigables navigablesThatNeedBeforeUnload, given an optional traversable navigable traversable, an optional integer targetStep, and an optional user navigation involvement-or-null userInvolvementForNavigateEvent, run these steps. They return "canceled-by-beforeunload", "canceled-by-navigate", or "continue".

Let documentsToFireBeforeunload be the active document of each item in navigablesThatNeedBeforeUnload.
Let unloadPromptShown be false.
Let finalStatus be "continue".
If traversable was given, then:
1. Assert: targetStep and userInvolvementForNavigateEvent were given.
2. Let targetEntry be the result of getting the target history entry given traversable and targetStep.
3. If targetEntry is not traversable's current session history entry, and targetEntry's document state's origin is the same as traversable's current session history entry's document state's origin, then:
  
  In this case, we're going to fire the navigate event for traversable here. Because under some circumstances it might be canceled, we need to do this separately from other traversal navigate events, which happen later.
  
  Additionally, because we want beforeunload events to fire before navigate events, this means we need to fire beforeunload for traversable here (if applicable), instead of doing it as part of the below loop over documentsToFireBeforeunload.
  1. Assert: userInvolvementForNavigateEvent is not null.
  2. Let eventsFired be false.
  3. Let needsBeforeunload be true if navigablesThatNeedBeforeUnload contains traversable; otherwise false.
  4. If needsBeforeunload is true, then remove traversable's active document from documentsToFireBeforeunload.
  5. Queue a global task on the navigation and traversal task source given traversable's active window to perform the following steps:
    1. If needsBeforeunload is true, then:
      1. Let (unloadPromptShownForThisDocument, unloadPromptCanceledByThisDocument) be the result of running the steps to fire beforeunload given traversable's active document and false.
      2. If unloadPromptShownForThisDocument is true, then set unloadPromptShown to true.
      3. If unloadPromptCanceledByThisDocument is true, then set finalStatus to "canceled-by-beforeunload".
    2. If finalStatus is "canceled-by-beforeunload", then abort these steps.
    3. Let navigation be traversable's active window's navigation API.
    4. Let navigateEventResult be the result of firing a traverse navigate event at navigation given targetEntry and userInvolvementForNavigateEvent.
    5. If navigateEventResult is false, then set finalStatus to "canceled-by-navigate".
    6. Set eventsFired to true.
  6. Wait until eventsFired is true.
  7. If finalStatus is not "continue", then return finalStatus.
Let totalTasks be the size of documentsThatNeedBeforeunload.
Let completedTasks be 0.
For each document of documents, queue a global task on the navigation and traversal task source given document's relevant global object to run the steps:
1. Let (unloadPromptShownForThisDocument, unloadPromptCanceledByThisDocument) be the result of running the steps to fire beforeunload given document and unloadPromptShown.
2. If unloadPromptShownForThisDocument is true, then set unloadPromptShown to true.
3. If unloadPromptCanceledByThisDocument is true, then set finalStatus to "canceled-by-beforeunload".
4. Increment completedTasks.
Wait for completedTasks to be totalTasks.
Return finalStatus.

The steps to fire beforeunload given a Document document and a boolean unloadPromptShown are:

Let unloadPromptCanceled be false.
Increase the document's unload counter by 1.
Increase document's relevant agent's event loop's termination nesting level by 1.
Let eventFiringResult be the result of firing an event named beforeunload at document's relevant global object, using BeforeUnloadEvent, with the cancelable attribute initialized to true.
Decrease document's relevant agent's event loop's termination nesting level by 1.
If all of the following are true:
- unloadPromptShown is false;
- document's active sandboxing flag set does not have its sandboxed modals flag set;
- document's relevant global object has sticky activation;
- eventFiringResult is false, or the returnValue attribute of event is not the empty string; and
- showing an unload prompt is unlikely to be annoying, deceptive, or pointless,
then:
1. Set unloadPromptShown to true.
2. Let userPromptHandler be the result of WebDriver BiDi user prompt opened with document's relevant global object, "beforeunload", and "".
3. If userPromptHandler is "dismiss", then set unloadPromptCanceled to true.
4. If userPromptHandler is "none", then:
  - Ask the user to confirm that they wish to unload the document, and pause while waiting for the user's response.
    
    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.
  - If the user did not confirm the page navigation, then set unloadPromptCanceled to true.
5. Invoke WebDriver BiDi user prompt closed with document's relevant global object, "beforeunload", and true if unloadPromptCanceled is false or false otherwise.
Decrease document's unload counter by 1.
Return (unloadPromptShown, unloadPromptCanceled).

Each navigable has an ongoing navigation, which is a navigation ID, "traversal", or null, initially null. It is used to track navigation aborting and to prevent any navigations from taking place during traversal.

To set the ongoing navigation for a navigable navigable to newValue:

If navigable's ongoing navigation is equal to newValue, then return.
Inform the navigation API about aborting navigation given navigable.
Set navigable's ongoing navigation to newValue.

7.4.3 Reloading and traversing

To reload a navigable navigable given an optional serialized state-or-null navigationAPIState (default null) and an optional user navigation involvement userInvolvement (default "none"):

If userInvolvement is not "browser UI", then:
1. Let navigation be navigable's active window's navigation API.
2. Let destinationNavigationAPIState be navigable's active session history entry's navigation API state.
3. If navigationAPIState is not null, then set destinationNavigationAPIState to navigationAPIState.
4. Let continue be the result of firing a push/replace/reload navigate event at navigation with navigationType set to "reload", isSameDocument set to false, userInvolvement set to userInvolvement, destinationURL set to navigable's active session history entry's URL, and navigationAPIState set to destinationNavigationAPIState.
5. If continue is false, then return.
Set navigable's active session history entry's document state's reload pending to true.
Let traversable be navigable's traversable navigable.
Append the following session history traversal steps to traversable:
1. Apply the reload history step to traversable.

To traverse the history by a delta given a traversable navigable traversable, an integer delta, and an optional Document sourceDocument:

Let sourceSnapshotParams and initiatorToCheck be null.
Let userInvolvement be "browser UI".
If sourceDocument is given, then:
1. Set sourceSnapshotParams to the result of snapshotting source snapshot params given sourceDocument.
2. Set initiatorToCheck to sourceDocument's node navigable.
3. Set userInvolvement to "none".
Append the following session history traversal steps to traversable:
1. Let allSteps be the result of getting all used history steps for traversable.
2. Let currentStepIndex be the index of traversable's current session history step within allSteps.
3. Let targetStepIndex be currentStepIndex plus delta.
4. If allSteps[targetStepIndex] does not exist, then abort these steps.
5. Apply the traverse history step allSteps[targetStepIndex] to traversable, given sourceSnapshotParams, initiatorToCheck, and userInvolvement.

7.4.4 Non-fragment synchronous "navigations"

Apart from the navigate algorithm, session history entries can be pushed or replaced via one more mechanism, the URL and history update steps. The most well-known callers of these steps are the history.replaceState() and history.pushState() APIs, but various other parts of the standard also need to perform updates to the active history entry, and they use these steps to do so.

The URL and history update steps, given a Document document, a URL newURL, an optional serialized state-or-null serializedData (default null), and an optional history handling behavior historyHandling (default "replace"), are:

Let navigable be document's node navigable.
Let activeEntry be navigable's active session history entry.
Let newEntry be a new session history entry, with

URL
newURL
serialized state
if serializedData is not null, serializedData; otherwise activeEntry's classic history API state
document state
activeEntry's document state
scroll restoration mode
activeEntry's scroll restoration mode
persisted user state
activeEntry's persisted user state
If document's is initial about:blank is true, then set historyHandling to "replace".

This means that pushState() on an initial about:blank Document behaves as a replaceState() call.
Let entryToReplace be activeEntry if historyHandling is "replace", otherwise null.
If historyHandling is "push", then:
1. Increment document's history object's index.
2. Set document's history object's length to its index + 1.
These are temporary best-guess values for immediate synchronous access.
If serializedData is not null, then restore the history object state given document and newEntry.
Set document's URL to newURL.

Since this is neither a navigation nor a history traversal, it does not cause a hashchange event to be fired.
Set document's latest entry to newEntry.
Set navigable's active session history entry to newEntry.
Update the navigation API entries for a same-document navigation given document's relevant global object's navigation API, newEntry, and historyHandling.
Let traversable be navigable's traversable navigable.
Append the following session history synchronous navigation steps involving navigable to traversable:
1. Finalize a same-document navigation given traversable, navigable, newEntry, entryToReplace, and historyHandling.
2. Invoke WebDriver BiDi history updated with navigable.

Although both fragment navigation and the URL and history update steps perform synchronous history updates, only fragment navigation contains a synchronous call to update document for history step application. The URL and history update steps instead perform a few select updates inside the above algorithm, omitting others. This is somewhat of an unfortunate historical accident, and generally leads to web-developer sadness about the inconsistency. For example, this means that popstate events fire for fragment navigations, but not for history.pushState() calls.

7.4.5 Populating a session history entry

As explained in the overview, both navigation and traversal involve creating a session history entry and then attempting to populate its document member, so that it can be presented inside the navigable.

This involves either: using an already-given response; using the srcdoc resource stored in the session history entry; or fetching. The process has several failure modes, which can either result in doing nothing (leaving the navigable on its currently-active Document) or can result in populating the session history entry with an error document.

To attempt to populate the history entry's document for a session history entry entry, given a navigable navigable, a NavigationTimingType navTimingType, a source snapshot params sourceSnapshotParams, a target snapshot params targetSnapshotParams, an optional navigation ID-or-null navigationId (default null), an optional navigation params-or-null navigationParams (default null), an optional string cspNavigationType (default "other"), an optional boolean allowPOST (default false), and optional algorithm steps completionSteps (default an empty algorithm):

Assert: this is running in parallel.
Assert: if navigationParams is non-null, then navigationParams's response is non-null.
Let currentBrowsingContext be navigable's active browsing context.
Let documentResource be entry's document state's resource.
If navigationParams is null, then:
1. If documentResource is a string, then set navigationParams to the result of creating navigation params from a srcdoc resource given entry, navigable, targetSnapshotParams, navigationId, and navTimingType.
2. Otherwise, if all of the following are true:
  - entry's URL's scheme is a fetch scheme; and
  - documentResource is null, or allowPOST is true and documentResource's request body is not failure,
  then set navigationParams to the result of creating navigation params by fetching given entry, navigable, sourceSnapshotParams, targetSnapshotParams, cspNavigationType, navigationId, and navTimingType.
3. Otherwise, if entry's URL's scheme is not a fetch scheme, then set navigationParams to a new non-fetch scheme navigation params, with
  
  id
  navigationId
  navigable
  navigable
  URL
  entry's URL
  target snapshot sandboxing flags
  targetSnapshotParams's sandboxing flags
  source snapshot has transient activation
  sourceSnapshotParams's has transient activation
  initiator origin
  entry's document state's initiator origin
  navigation timing type
  navTimingType
Queue a global task on the navigation and traversal task source, given navigable's active window, to run these steps:
1. If navigable's ongoing navigation no longer equals navigationId, then run completionSteps and abort these steps.
2. Let saveExtraDocumentState be true.
  
  Usually, in the cases where we end up populating entry's document state's document, we then want to save some of the state from that Document into entry. This ensures that if there are future traversals to entry where its document has been destroyed, we can use that state when creating a new Document.
  
  However, in some specific cases, saving the state would be unhelpful. For those, we set saveExtraDocumentState to false later in this algorithm.
3. If navigationParams is a non-fetch scheme navigation params, then:
  1. Set entry's document state's document to the result of running attempt to create a non-fetch scheme document given navigationParams.
    
    This can result in setting entry's document state's document to null, e.g., when handing-off to external software.
  2. Set saveExtraDocumentState to false.
4. Otherwise, if any of the following are true:
  - navigationParams is null;
  - the result of should navigation response to navigation request of type in target be blocked by Content Security Policy? given navigationParams's request, navigationParams's response, navigationParams's policy container's CSP list, cspNavigationType, and navigable is "Blocked";
  - navigationParams's reserved environment is non-null and the result of checking a navigation response's adherence to its embedder policy given navigationParams's response, navigable, and navigationParams's policy container's embedder policy is false; or
  - the result of checking a navigation response's adherence to `X-Frame-Options` given navigationParams's response, navigable, navigationParams's policy container's CSP list, and navigationParams's origin is false,
  then:
  1. Set entry's document state's document to the result of creating a document for inline content that doesn't have a DOM, given navigable, null, and navTimingType. The inline content should indicate to the user the sort of error that occurred.
  2. Make document unsalvageable given entry's document state's document and "navigation-failure".
  3. Set saveExtraDocumentState to false.
  4. If navigationParams is not null, then:
    1. Run the environment discarding steps for navigationParams's reserved environment.
    2. Invoke WebDriver BiDi navigation failed with currentBrowsingContext and a new WebDriver BiDi navigation status whose id is navigationId, status is "canceled", and url is navigationParams's response's URL.
5. Otherwise, if navigationParams's response has a `Content-Disposition` header specifying the attachment disposition type, then:
  1. Let sourceAllowsDownloading be sourceSnapshotParams's allows downloading.
  2. Let targetAllowsDownloading be false if navigationParams's final sandboxing flag set has the sandboxed downloads browsing context flag set; otherwise true.
  3. Let uaAllowsDownloading be true.
  4. Optionally, the user agent may set uaAllowsDownloading to false, if it believes doing so would safeguard the user from a potentially hostile download.
  5. If sourceAllowsDownloading, targetAllowsDownloading, and uaAllowsDownloading are true, then:
    1. Handle navigationParams's response as a download.
    2. Invoke WebDriver BiDi download started with currentBrowsingContext and a new WebDriver BiDi navigation status whose id is navigationId, status is "complete", and url is navigationParams's response's URL.
  This branch leaves entry's document state's document as null.
6. Otherwise, if navigationParams's response's status is not 204 and is not 205, then set entry's document state's document to the result of loading a document given navigationParams, sourceSnapshotParams, and entry's document state's initiator origin.
  
  This can result in setting entry's document state's document to null, e.g., when handing-off to external software.
7. If entry's document state's document is not null, then:
  1. Set entry's document state's ever populated to true.
  2. If saveExtraDocumentState is true:
    1. Let document be entry's document state's document.
    2. Set entry's document state's origin to document's origin.
    3. If document's URL requires storing the policy container in history, then:
      1. Assert: navigationParams is a navigation params (i.e., neither null nor a non-fetch scheme navigation params).
      2. Set entry's document state's history policy container to navigationParams's policy container.
  3. If entry's document state's request referrer is "client", and navigationParams is a navigation params (i.e., neither null nor a non-fetch scheme navigation params), then:
    1. Assert: navigationParams's request is not null.
    2. Set entry's document state's request referrer to navigationParams's request's referrer.
8. Run completionSteps.

To create navigation params from a srcdoc resource given a session history entry entry, a navigable navigable, a target snapshot params targetSnapshotParams, a navigation ID-or-null navigationId, and a NavigationTimingType navTimingType:

Let documentResource be entry's document state's resource.
Let response be a new response with

URL
about:srcdoc
header list
« (`Content-Type`, `text/html`) »
body
the UTF-8 encoding of documentResource, as a body
Let responseOrigin be the result of determining the origin given response's URL, targetSnapshotParams's sandboxing flags, and entry's document state's origin.
Let coop be a new opener policy.
Let coopEnforcementResult be a new opener policy enforcement result with

url
response's URL
origin
responseOrigin
opener policy
coop
Let policyContainer be the result of determining navigation params policy container given response's URL, entry's document state's history policy container, null, navigable's container document's policy container, and null.
Return a new navigation params, with

id
navigationId
navigable
navigable
request
null
response
response
fetch controller
null
commit early hints
null
COOP enforcement result
coopEnforcementResult
reserved environment
null
origin
responseOrigin
policy container
policyContainer
final sandboxing flag set
targetSnapshotParams's sandboxing flags
opener policy
coop
navigation timing type
navTimingType
about base URL
entry's document state's about base URL

To create navigation params by fetching given a session history entry entry, a navigable navigable, a source snapshot params sourceSnapshotParams, a target snapshot params targetSnapshotParams, a string cspNavigationType, a navigation ID-or-null navigationId, and a NavigationTimingType navTimingType, perform the following steps. They return a navigation params, a non-fetch scheme navigation params, or null.

This algorithm mutates entry.

Assert: this is running in parallel.
Let documentResource be entry's document state's resource.
Let request be a new request, with

url
entry's URL
client
sourceSnapshotParams's fetch client
destination
"document"
credentials mode
"include"
use-URL-credentials flag
set
redirect mode
"manual"
replaces client id
navigable's active document's relevant settings object's id
mode
"navigate"
referrer
entry's document state's request referrer
referrer policy
entry's document state's request referrer policy
If documentResource is a POST resource, then:
1. Set request's method to `POST`.
2. Set request's body to documentResource's request body.
3. Set `Content-Type` to documentResource's request content-type in request's header list.
If entry's document state's reload pending is true, then set request's reload-navigation flag.
Otherwise, if entry's document state's ever populated is true, then set request's history-navigation flag.
If sourceSnapshotParams's has transient activation is true, then set request's user-activation to true.
If navigable's container is non-null:
1. If the navigable'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 navigable's container's local name.
3. If sourceSnapshotParams's fetch client is navigable's container document's relevant settings object, then set request's initiator type to navigable's container's local name.
  
  This ensure that only container-initiated navigations are reported to resource timing.
Let response be null.
Let responseOrigin be null.
Let fetchController be null.
Let coopEnforcementResult be a new opener policy enforcement result, with

url
navigable's active document's URL
origin
navigable's active document's origin
opener policy
navigable's active document's opener policy
current context is navigation source
true if navigable's active document's origin is same origin with entry's document state's initiator origin otherwise false
Let finalSandboxFlags be an empty sandboxing flag set.
Let responsePolicyContainer be null.
Let responseCOOP be a new opener policy.
Let locationURL be null.
Let currentURL be request's current URL.
Let commitEarlyHints be null.
While true:
1. If request's reserved client is not null and currentURL's origin is not the same as request's reserved client's creation URL's origin, then:
  1. Run the environment discarding steps for request's reserved client.
  2. Set request's reserved client to null.
  3. Set commitEarlyHints to null.
    
    Preloaded links from early hint headers remain in the preload cache after a same origin redirect, but get discarded when the redirect is cross-origin.
2. If request's reserved client is null, then:
  1. Let topLevelCreationURL be currentURL.
  2. Let topLevelOrigin be null.
  3. If navigable is not a top-level traversable, then:
    1. Let parentEnvironment be navigable's parent's active document's relevant settings object.
    2. Set topLevelCreationURL to parentEnvironment's top-level creation URL.
    3. Set topLevelOrigin to parentEnvironment's top-level origin.
  4. Set request's reserved client to a new environment whose id is a unique opaque string, target browsing context is navigable's active browsing context, 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]
3. If the result of should navigation request of type be blocked by Content Security Policy? given request and cspNavigationType is "Blocked", then set response to a network error and break. [CSP]
4. Set response to null.
5. If fetchController is null, then set fetchController to the result of fetching request, with processEarlyHintsResponse set to processEarlyHintsResponse as defined below, processResponse set to processResponse as defined below, and useParallelQueue set to true.
  
  Let processEarlyHintsResponse be the following algorithm given a response earlyResponse:
  1. If commitEarlyHints is null, then set commitEarlyHints to the result of processing early hint headers given earlyResponse and request's reserved client.
  Let processResponse be the following algorithm given a response fetchedResponse:
  1. Set response to fetchedResponse.
6. Otherwise, process the next manual redirect for fetchController.
  
  This will result in calling the processResponse we supplied above, during our first iteration through the loop, and thus setting response.
  
  Navigation handles redirects manually as navigation is the only place in the web platform that cares for redirects to mailto: URLs and such.
7. Wait until either response is non-null, or navigable's ongoing navigation changes to no longer equal navigationId.
  
  If the latter condition occurs, then abort fetchController, and return.
  
  Otherwise, proceed onward.
8. If request's body is null, then set entry's document state's resource to null.
  
  Fetch unsets the body for particular redirects.
9. Set responsePolicyContainer to the result of creating a policy container from a fetch response given response and request's reserved client.
10. Set finalSandboxFlags to the union of targetSnapshotParams's sandboxing flags and responsePolicyContainer's CSP list's CSP-derived sandboxing flags.
11. Set responseOrigin to the result of determining the origin given response's URL, finalSandboxFlags, and entry's document state's initiator origin.
  
  If response is a redirect, then response's URL will be the URL that led to the redirect to response's location URL; it will not be the location URL itself.
12. If navigable is a top-level traversable, then:
  1. Set responseCOOP to the result of obtaining an opener policy given response and request's reserved client.
  2. Set coopEnforcementResult to the result of enforcing the response's opener policy given navigable's active browsing context, response's URL, responseOrigin, responseCOOP, coopEnforcementResult and request's referrer.
  3. If finalSandboxFlags is not empty and responseCOOP's value 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 opener policy and sandbox the result of navigating to that response.
13. If response is not a network error, navigable is a child navigable, and the result of performing a cross-origin resource policy check with navigable's container document's origin, navigable'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 navigable rather than navigable itself. This is because we care about the same-originness of the embedded content against the parent context, not the navigation source.
14. Set locationURL to response's location URL given currentURL's fragment.
15. If locationURL is failure or null, then break.
16. Assert: locationURL is a URL.
17. Set entry's classic history API state to StructuredSerializeForStorage(null).
18. Let oldDocState be entry's document state.
19. Set entry's document state to a new document state, with
  history policy container
  a clone of the oldDocState's history policy container if it is non-null; null otherwise
  request referrer
  oldDocState's request referrer
  request referrer policy
  oldDocState's request referrer policy
  initiator origin
  oldDocState's initiator origin
  origin
  oldDocState's origin
  about base URL
  oldDocState's about base URL
  resource
  oldDocState's resource
  ever populated
  oldDocState's ever populated
  navigable target name
  oldDocState's navigable target name
  
  For the navigation case, only entry referenced oldDocState, which was created early in the navigate algorithm. So for navigations, this is functionally just an update to entry's document state. For the traversal case, it's possible adjacent session history entries also reference oldDocState, in which case they will continue doing so even after we've updated entry's document state.
  
  oldDocState's history policy container is only ever non-null here in the traversal case, after we've populated it during a navigation to a URL that requires storing the policy container in history.
  
  The setup is given by the following Jake diagram:
  
  0 1 2 3
  top /a /a#foo /a#bar /b
  
  Also assume that the document state shared by the entries in steps 0, 1, and 2 has a null document, i.e., bfcache is not in play.
  
  Now consider the scenario where we traverse back to step 2, but this time when fetching /a, the server responds with a `Location` header pointing to /c. That is, locationURL points to /c and so we have reached this step instead of breaking out of the loop.
  
  In this case, we replace the document state of the session history entry occupying step 2, but we do not replace the document state of the entries occupying steps 0 and 1. The resulting Jake diagram looks like this:
  
  0 1 2 3
  top /a /a#foo /c#bar /b
  
  Note that we perform this replacement even if we end up in a redirect chain back to the original URL, for example if /c itself had a `Location` header pointing to /a. Such a case would end up like so:
  
  0 1 2 3
  top /a /a#foo /a#bar /b
20. If locationURL's scheme is not an HTTP(S) scheme, then:
  1. Set entry's document state's resource to null.
  2. Break.
21. Set currentURL to locationURL.
22. Set entry's URL to currentURL.
By the end of this loop we will be in one of these scenarios:
- locationURL is failure, because of an unparseable `Location` header.
- locationURL is null, either because response is a network error or because we successfully fetched a non-network error HTTP(S) response with no `Location` header.
- locationURL is a URL with a non-HTTP(S) scheme.
If locationURL is a URL whose scheme is not a fetch scheme, then return a new non-fetch scheme navigation params, with

id
navigationId
navigable
navigable
URL
locationURL
target snapshot sandboxing flags
targetSnapshotParams's sandboxing flags
source snapshot has transient activation
sourceSnapshotParams's has transient activation
initiator origin
responseOrigin
navigation timing type
navTimingType

At this point, request's current URL is the last URL in the redirect chain with a fetch scheme before redirecting to a non-fetch scheme URL. It is this URL's origin that will be used as the initiator origin for navigations to non-fetch scheme URLs.
If any of the following are true:
- response is a network error;
- locationURL is failure; or
- locationURL is a URL whose scheme is a fetch scheme,
then return null.

We allow redirects to non-fetch scheme URLs, but redirects to fetch scheme URLs that aren't HTTP(S) are treated like network errors.
Assert: locationURL is null and response is not a network error.
Let resultPolicyContainer be the result of determining navigation params policy container given response's URL, entry's document state's history policy container, sourceSnapshotParams's source policy container, null, and responsePolicyContainer.
If navigable's container is an iframe, and response's timing allow passed flag is set, then set container's pending resource-timing start time to null.

If the iframe is allowed to report to resource timing, we don't need to run its fallback steps as the normal reporting would happen.
Return a new navigation params, with

id
navigationId
navigable
navigable
request
request
response
response
fetch controller
fetchController
commit early hints
commitEarlyHints
opener policy
responseCOOP
reserved environment
request's reserved client
origin
responseOrigin
policy container
resultPolicyContainer
final sandboxing flag set
finalSandboxFlags
COOP enforcement result
coopEnforcementResult
navigation timing type
navTimingType
about base URL
entry's document state's about base URL

	0	1	2	3
`top`	/a	/a#foo	/a#bar	/b

	0	1	2	3
`top`	/a	/a#foo	/c#bar	/b

	0	1	2	3
`top`	/a	/a#foo	/a#bar	/b

An element has a browsing context scope origin if its Document's node navigable is a top-level traversable or if all of its Document's ancestor navigables all have active documents whose origins are the same origin as the element's node document's origin. If an element has a browsing context scope origin, then its value is the origin of the element's node document.

This definition is broken and needs investigation to see what it was intended to express: see issue #4703.

To load a document given navigation params navigationParams, source snapshot params sourceSnapshotParams, and origin initiatorOrigin, perform the following steps. They return a Document or null.

Let type be the computed type of navigationParams's response.
If the user agent has been configured to process resources of the given type using some mechanism other than rendering the content in a navigable, then skip this step. Otherwise, if the type is one of the following types:

an HTML MIME type
Return the result of loading an HTML document, given navigationParams.
an XML MIME type that is not an explicitly supported XML MIME type
Return the result of loading an XML document given navigationParams and type.
a JavaScript MIME type
a JSON MIME type that is not an explicitly supported JSON MIME type
"text/css"
"text/plain"
"text/vtt"
Return the result of loading a text document given navigationParams and type.
"multipart/x-mixed-replace"
Return the result of loading a multipart/x-mixed-replace document, given navigationParams, sourceSnapshotParams, and initiatorOrigin.
A supported image, video, or audio type
Return the result of loading a media document given navigationParams and type.
"application/pdf"
"text/pdf"
If the user agent's PDF viewer supported is true, return the result of creating a document for inline content that doesn't have a DOM given navigationParams's navigable.

Otherwise, proceed onward.

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, or for which the user agent has dedicated processing rules. For example, a web browser with a built-in Atom feed viewer would be said to explicitly support the application/atom+xml MIME type.

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, or for which the user agent has dedicated processing rules.

In both cases, the external application or user agent will either display the content inline directly in navigationParams's navigable, or hand it off to external software. Both happen in the steps below.
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 return the result of creating a document for inline content that doesn't have a DOM given navigationParams's navigable, navigationParams's id, and navigationParams's navigation timing type.
Otherwise, the document's type is such that the resource will not affect navigationParams's navigable, 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. Hand-off to external software given navigationParams's response, navigationParams's navigable, navigationParams's final sandboxing flag set, sourceSnapshotParams's has transient activation, and initiatorOrigin.
Return null.

7.4.6 Applying the history step

For both navigation and traversal, once we have an idea of where we want to head to in the session history, much of the work comes about in applying that notion to the traversable navigable and the relevant Document. For navigations, this work generally occurs toward the end of the process; for traversals, it is the beginning.

7.4.6.1 Updating the traversable

Ensuring a traversable ends up at the right session history step is particularly complex, as it can involve coordinating across multiple navigable descendants of the traversable, populating them in parallel, and then synchronizing back up to ensure everyone has the same view of the result. This is further complicated by the existence of synchronous same-document navigations being mixed together with cross-document navigations, and how web pages have come to have certain relative timing expectations.

A changing navigable continuation state is used to store information during the apply the history step algorithm, allowing parts of the algorithm to continue only after other parts have finished. It is a struct with:

displayed document: A Document
target entry: A session history entry
navigable: A navigable
update only: A boolean

Although all updates to the traversable navigable end up in the same apply the history step algorithm, each possible entry point comes along with some minor customizations:

To update for navigable creation/destruction given a traversable navigable traversable:

Let step be traversable's current session history step.
Return the result of applying the history step step to traversable given false, null, null, null, and null.

To apply the push/replace history step given a non-negative integer step and a history handling behavior historyHandling to a traversable navigable traversable:

Return the result of applying the history step step to traversable given false, null, null, null, and historyHandling.

Apply the push/replace history step never passes source snapshot params or an initiator navigable to apply the history step. This is because those checks are done earlier in the navigation algorithm.

To apply the reload history step to a traversable navigable traversable:

Let step be traversable's current session history step.
Return the result of applying the history step step to traversable given true, null, null, null, and "reload".

Apply the reload history step never passes source snapshot params or an initiator navigable to apply the history step. This is because reloading is always treated as if it were done by the navigable itself, even in cases like parent.location.reload().

To apply the traverse history step given a non-negative integer step to a traversable navigable traversable, with source snapshot params sourceSnapshotParams, navigable initiatorToCheck, and user navigation involvement userInvolvement:

Return the result of applying the history step step to traversable given true, sourceSnapshotParams, initiatorToCheck, userInvolvement, and "traverse".

Now for the algorithm itself.

To apply the history step given a non-negative integer step to a traversable navigable traversable, with boolean checkForCancelation, source snapshot params-or-null sourceSnapshotParams, navigable-or-null initiatorToCheck, user navigation involvement-or-null userInvolvementForNavigateEvents, and NavigationType-or-null navigationType, perform the following steps. They return "initiator-disallowed", "canceled-by-beforeunload", "canceled-by-navigate", or "applied".

Assert: This is running within traversable's session history traversal queue.
Let targetStep be the result of getting the used step given traversable and step.
If initiatorToCheck is not null, then:
1. Assert: sourceSnapshotParams is not null.
2. For each navigable of get all navigables whose current session history entry will change or reload: if initiatorToCheck is not allowed by sandboxing to navigate navigable given sourceSnapshotParams, then return "initiator-disallowed".
Let navigablesCrossingDocuments be the result of getting all navigables that might experience a cross-document traversal given traversable and targetStep.
If checkForCancelation is true, and the result of checking if unloading is canceled given navigablesCrossingDocuments, traversable, targetStep, and userInvolvementForNavigateEvents is not "continue", then return that result.
Let changingNavigables be the result of get all navigables whose current session history entry will change or reload given traversable and targetStep.
Let nonchangingNavigablesThatStillNeedUpdates be the result of getting all navigables that only need history object length/index update given traversable and targetStep.
For each navigable of changingNavigables:
1. Let targetEntry be the result of getting the target history entry given navigable and targetStep.
2. Set navigable's current session history entry to targetEntry.
3. Set the ongoing navigation for navigable to "traversal".
Let totalChangeJobs be the size of changingNavigables.
Let completedChangeJobs be 0.
Let changingNavigableContinuations be an empty queue of changing navigable continuation states.

This queue is used to split the operations on changingNavigables into two parts. Specifically, changingNavigableContinuations holds data for the second part.
For each navigable of changingNavigables, queue a global task on the navigation and traversal task source of navigable's active window to run the steps:

This set of steps are split into two parts to allow synchronous navigations to be processed before documents unload. State is stored in changingNavigableContinuations for the second part.
1. Let displayedEntry be navigable's active session history entry.
2. Let targetEntry be navigable's current session history entry.
3. Let changingNavigableContinuation be a changing navigable continuation state with:
  
  displayed document
  displayedEntry's document
  target entry
  targetEntry
  navigable
  navigable
  update-only
  false
4. If displayedEntry is targetEntry and targetEntry's document state's reload pending is false, then:
  1. Set changingNavigableContinuation's update-only to true.
  2. Enqueue changingNavigableContinuation on changingNavigableContinuations.
  3. Abort these steps.
  This case occurs due to a synchronous navigation which already updated the active session history entry.
5. Switch on navigationType:
  
  "reload"
  Assert: targetEntry's document state's reload pending is true.
  "traverse"
  Assert: targetEntry's document state's ever populated is true.
  "replace"
  Assert: targetEntry's step is displayedEntry's step and targetEntry's document state's ever populated is false.
  "push"
  Assert: targetEntry's step is displayedEntry's step + 1 and targetEntry's document state's ever populated is false.
6. Let oldOrigin be targetEntry's document state's origin.
7. If all of the following are true:
  - navigable is not traversable;
  - targetEntry is not navigable's current session history entry; and
  - oldOrigin is the same as navigable's current session history entry's document state's origin,
  then:
  1. Assert: userInvolvementForNavigateEvents is not null.
  2. Let navigation be navigable's active window's navigation API.
  3. Fire a traverse navigate event at navigation given targetEntry and userInvolvementForNavigateEvents.
8. If targetEntry's document is null, or targetEntry's document state's reload pending is true, then:
  1. Let navTimingType be "back_forward" if targetEntry's document is null; otherwise "reload".
  2. Let targetSnapshotParams be the result of snapshotting target snapshot params given navigable.
  3. Let potentiallyTargetSpecificSourceSnapshotParams be sourceSnapshotParams.
  4. If potentiallyTargetSpecificSourceSnapshotParams is null, then set it to the result of snapshotting source snapshot params given navigable's active document.
    
    In this case there is no clear source of the traversal/reload. We treat this situation as if navigable navigated itself, but note that some properties of targetEntry's original initiator are preserved in targetEntry's document state, such as the initiator origin and referrer, which will appropriately influence the navigation.
  5. Set targetEntry's document state's reload pending to false.
  6. Let allowPOST be targetEntry's document state's reload pending.
  7. In parallel, attempt to populate the history entry's document for targetEntry, given navigable, potentiallyTargetSpecificSourceSnapshotParams, targetSnapshotParams, with allowPOST set to allowPOST and completionSteps set to queue a global task on the navigation and traversal task source given navigable's active window to run afterDocumentPopulated.
  Otherwise, run afterDocumentPopulated immediately.
  
  In both cases, let afterDocumentPopulated be the following steps:
  1. If targetEntry's document is null, then set changingNavigableContinuation's update-only to true.
    
    This means we tried to populate the document, but were unable to do so, e.g. because of the server returning a 204.
    
    These kinds of failed navigations or traversals will not be signaled to the navigation API (e.g., through the promises of any navigation API method tracker, or the navigateerror event). Doing so would leak information about the timing of responses from other origins, in the cross-origin case, and providing different results in the cross-origin vs. same-origin cases was deemed too confusing.
    
    However, implementations could use this opportunity to clear any promise handlers for the navigation.transition.finished promise, as they are guaranteed at this point to never run. And, they might wish to report a warning to the console if any part of the navigation API initiated these navigations, to make it clear to the web developer why their promises will never settle and events will never fire.
  2. If targetEntry's document's origin is not oldOrigin, then set targetEntry's classic history API state to StructuredSerializeForStorage(null).
    
    This clears history state when the origin changed vs a previous load of targetEntry without a redirect occuring. This can happen due to a change in CSP sandbox headers.
  3. If all of the following are true:
    - navigable's parent is null;
    - targetEntry's document's browsing context is not an auxiliary browsing context whose opener browsing context is non-null; and
    - targetEntry's document's origin is not oldOrigin,
    then set targetEntry's document state's navigable target name to the empty string.
  4. Enqueue changingNavigableContinuation on changingNavigableContinuations.
    
    The rest of this job runs later in this algorithm.
Let navigablesThatMustWaitBeforeHandlingSyncNavigation be an empty set.
While completedChangeJobs does not equal totalChangeJobs:
1. If traversable's running nested apply history step is false, then:
  1. While traversable's session history traversal queue's algorithm set contains one or more synchronous navigation steps with a target navigable not contained in navigablesThatMustWaitBeforeHandlingSyncNavigation:
    1. Let steps be the first item in traversable's session history traversal queue's algorithm set that is synchronous navigation steps with a target navigable not contained in navigablesThatMustWaitBeforeHandlingSyncNavigation.
    2. Remove steps from traversable's session history traversal queue's algorithm set.
    3. Set traversable's running nested apply history step to true.
    4. Run steps.
    5. Set traversable's running nested apply history step to false.
    Synchronous navigations that are intended to take place before this traversal jump the queue at this point, so they can be added to the correct place in traversable's session history entries before this traversal potentially unloads their document. More details can be found here.
2. Let changingNavigableContinuation be the result of dequeuing from changingNavigableContinuations.
3. If changingNavigableContinuation is nothing, then continue.
4. Let displayedDocument be changingNavigableContinuation's displayed document.
5. Let targetEntry be changingNavigableContinuation's target entry.
6. Let navigable be changingNavigableContinuation's navigable.
7. Let (scriptHistoryLength, scriptHistoryIndex) be the result of getting the history object length and index given traversable and targetStep.
  
  These values might have changed since they were last calculated.
8. Append navigable to navigablesThatMustWaitBeforeHandlingSyncNavigation.
  
  Once a navigable has reached this point in traversal, additionally queued synchronous navigation steps are likely to be intended to occur after this traversal rather than before it, so they no longer jump the queue. More details can be found here.
9. Let entriesForNavigationAPI be the result of getting session history entries for the navigation API given navigable and targetStep.
10. If changingNavigableContinuation's update-only is true, or targetEntry's document is displayedDocument, then:
  
  This is a same-document navigation: we proceed without unloading.
  1. Set the ongoing navigation for navigable to null.
    
    This allows new navigations of navigable to start, whereas during the traversal they were blocked.
  2. Queue a global task on the navigation and traversal task source given navigable's active window to perform afterPotentialUnloads.
11. Otherwise:
  1. Assert: navigationType is not null.
  2. Deactivate displayedDocument, given userNavigationInvolvement, targetEntry, navigationType, and afterPotentialUnloads.
12. In both cases, let afterPotentialUnloads be the following steps:
  1. Let previousEntry be navigable's active session history entry.
  2. If changingNavigableContinuation's update-only is false, then activate history entry targetEntry for navigable.
  3. Let updateDocument be an algorithm step which performs update document for history step application given targetEntry's document, targetEntry, changingNavigableContinuation's update-only, scriptHistoryLength, scriptHistoryIndex, navigationType, entriesForNavigationAPI, and previousEntry.
  4. If targetEntry's document is equal to displayedDocument, then perform updateDocument.
  5. Otherwise, queue a global task on the navigation and traversal task source given targetEntry's document's relevant global object to perform updateDocument.
  6. Increment completedChangeJobs.
Let totalNonchangingJobs be the size of nonchangingNavigablesThatStillNeedUpdates.

This step onwards deliberately waits for all the previous operations to complete, as they include processing synchronous navigations which will also post tasks to update history length and index.
Let completedNonchangingJobs be 0.
Let (scriptHistoryLength, scriptHistoryIndex) be the result of getting the history object length and index given traversable and targetStep.
For each navigable of nonchangingNavigablesThatStillNeedUpdates, queue a global task on the navigation and traversal task source given navigable's active window to run the steps:
1. Let document be navigable's active document.
2. Set document's history object's index to scriptHistoryIndex.
3. Set document's history object's length to scriptHistoryLength.
4. Increment completedNonchangingJobs.
Wait for completedNonchangingJobs to equal totalNonchangingJobs.
Set traversable's current session history step to targetStep.
Return "applied".

To deactivate a document for a cross-document navigation given a Document displayedDocument, a user navigation involvement userNavigationInvolvement, a session history entry targetEntry, a NavigationType navigationType, and afterPotentialUnloads, which is an algorithm that receives no arguments:

Let navigable be displayedDocument's node navigable.
Let potentiallyTriggerViewTransition be false.
Let isBrowserUINavigation be true if userNavigationInvolvement is "browser UI"; otherwise false.
Set potentiallyTriggerViewTransition to the result of calling can navigation trigger a cross-document view-transition? given displayedDocument, targetEntry's document, navigationType, and isBrowserUINavigation.
If potentiallyTriggerViewTransition is false, then:
1. Let firePageSwapBeforeUnload be the following step:
  1. Fire the pageswap event given displayedDocument, targetEntry, navigationType, and null.
2. Set the ongoing navigation for navigable to null.
  
  This allows new navigations of navigable to start, whereas during the traversal they were blocked.
3. Unload a document and its descendants given displayedDocument, targetEntry's document, afterPotentialUnloads, and firePageSwapBeforeUnload.
Otherwise, queue a global task on the navigation and traversal task source given navigable's active window to run the steps:
1. Let proceedWithNavigationAfterViewTransitionCapture be the following step:
  1. Append the following session history traversal steps to navigable's traversable navigable:
    1. Set the ongoing navigation for navigable to null.
      
      This allows new navigations of navigable to start, whereas during the traversal they were blocked.
    2. Unload a document and its descendants given displayedDocument, targetEntry's document, and afterPotentialUnloads.
2. Let viewTransition be the result of setting up a cross-document view-transition given displayedDocument, targetEntry's document, navigationType, and proceedWithNavigationAfterViewTransitionCapture.
3. Fire the pageswap event given displayedDocument, targetEntry, navigationType, and viewTransition.
4. If viewTransition is null, then run proceedWithNavigationAfterViewTransitionCapture.
  
  In the case where a view transition started, the view transitions algorithms are responsible for calling proceedWithNavigationAfterViewTransitionCapture.

To fire the pageswap event given a Document displayedDocument, a session history entry targetEntry, a NavigationType navigationType, and a ViewTransition-or-null viewTransition:

Assert: this is running as part of a task queued on displayedDocument's relevant agent's event loop.
Let navigation be displayedDocument's relevant global object's navigation API.
Let activation be null.
If all of the following are true:
- targetEntry's document's origin is same origin with displayedDocument's origin; and
- targetEntry's document's was created via cross-origin redirects is false, or targetEntry's document's latest entry is not null,
then:
1. Let destinationEntry be determined by switching on navigationType:
  
  "reload"
  The current entry of navigation
  "traverse"
  The NavigationHistoryEntry in navigation's entry list whose session history entry is targetEntry
  "push"
  "replace"
  A new NavigationHistoryEntry in displayedDocument's relevant realm with its session history entry set to targetEntry.
2. Set activation to a new NavigationActivation created in displayedDocument's relevant realm, with
  
  old entry
  the current entry of navigation
  new entry
  destinationEntry
  navigation type
  navigationType
This means that a cross-origin redirect during a navigation would result in a null activation in the old document's PageSwapEvent, unless the new document is being restored from bfcache.
Fire an event named pageswap at displayedDocument's relevant global object, using PageSwapEvent with its activation set to activation, and its viewTransition set to viewTransition.

To activate history entry session history entry entry for navigable navigable:

Save persisted state to the navigable's active session history entry.
Let newDocument be entry's document.
Assert: newDocument's is initial about:blank is false, i.e., we never traverse back to the initial about:blank Document because it always gets replaced when we navigate away from it.
Set navigable's active session history entry to entry.
Make active newDocument.

To get the used step given a traversable navigable traversable, and a non-negative integer step, perform the following steps. They return a non-negative integer.

Let steps be the result of getting all used history steps within traversable.
Return the greatest item in steps that is less than or equal to step.

This caters for situations where there's no session history entry with step step, due to the removal of a navigable.

To get the history object length and index given a traversable navigable traversable, and a non-negative integer step, perform the following steps. They return a tuple of two non-negative integers.

Let steps be the result of getting all used history steps within traversable.
Let scriptHistoryLength be the size of steps.
Assert: steps contains step.

It is assumed that step has been adjusted by getting the used step.
Let scriptHistoryIndex be the index of step in steps.
Return (scriptHistoryLength, scriptHistoryIndex).

To get all navigables whose current session history entry will change or reload given a traversable navigable traversable, and a non-negative integer targetStep, perform the following steps. They return a list of navigables.

Let results be an empty list.
Let navigablesToCheck be « traversable ».

This list is extended in the loop below.
For each navigable of navigablesToCheck:
1. Let targetEntry be the result of getting the target history entry given navigable and targetStep.
2. If targetEntry is not navigable's current session history entry or targetEntry's document state's reload pending is true, then append navigable to results.
3. If targetEntry's document is navigable's document, and targetEntry's document state's reload pending is false, then extend navigablesToCheck with the child navigables of navigable.
  
  Adding child navigables to navigablesToCheck means those navigables will also be checked by this loop. Child navigables are only checked if the navigable's active document will not change as part of this traversal.
Return results.

To get all navigables that only need history object length/index update given a traversable navigable traversable, and a non-negative integer targetStep, perform the following steps. They return a list of navigables.

Other navigables might not be impacted by the traversal. For example, if the response is a 204, the currently active document will remain. Additionally, going 'back' after a 204 will change the current session history entry, but the active session history entry will already be correct.

Let results be an empty list.
Let navigablesToCheck be « traversable ».

This list is extended in the loop below.
For each navigable of navigablesToCheck:
1. Let targetEntry be the result of getting the target history entry given navigable and targetStep.
2. If targetEntry is navigable's current session history entry and targetEntry's document state's reload pending is false, then:
  1. Append navigable to results.
  2. Extend navigablesToCheck with navigable's child navigables.
    
    Adding child navigables to navigablesToCheck means those navigables will also be checked by this loop. child navigables are only checked if the navigable's active document will not change as part of this traversal.
Return results.

To get the target history entry given a navigable navigable, and a non-negative integer step, perform the following steps. They return a session history entry.

Let entries be the result of getting session history entries for navigable.
Return the item in entries that has the greatest step less than or equal to step.

To see why getting the target history entry returns the entry with the greatest step less than or equal to the input step, consider the following Jake diagram:

	0	1	2	3
`top`	/t			/t#foo
`frames[0]`	/i-0-a	/i-0-b

For the input step 1, the target history entry for the top navigable is the /t entry, whose step is 0, while the target history entry for the frames[0] navigable is the /i-0-b entry, whose step is 1:

	0	1	2	3
`top`	/t			/t#foo
`frames[0]`	/i-0-a	/i-0-b

Similarly, given the input step 3 we get the top entry whose step is 3, and the frames[0] entry whose step is 1:

	0	1	2	3
`top`	/t			/t#foo
`frames[0]`	/i-0-a	/i-0-b

To get all navigables that might experience a cross-document traversal given a traversable navigable traversable, and a non-negative integer targetStep, perform the following steps. They return a list of navigables.

From traversable's session history traversal queue's perspective, these documents are candidates for going cross-document during the traversal described by targetStep. They will not experience a cross-document traversal if the status code for their target document is HTTP 204 No Content.

Note that if a given navigable might experience a cross-document traversal, this algorithm will return navigable but not its child navigables. Those would end up unloaded, not traversed.

Let results be an empty list.
Let navigablesToCheck be « traversable ».

This list is extended in the loop below.
For each navigable of navigablesToCheck:
1. Let targetEntry be the result of getting the target history entry given navigable and targetStep.
2. If targetEntry's document is not navigable's document or targetEntry's document state's reload pending is true, then append navigable to results.
  
  Although navigable's active history entry can change synchronously, the new entry will always have the same Document, so accessing navigable's document is reliable.
3. Otherwise, extend navigablesToCheck with navigable's child navigables.
  
  Adding child navigables to navigablesToCheck means those navigables will also be checked by this loop. Child navigables are only checked if the navigable's active document will not change as part of this traversal.
Return results.

7.4.6.2 Updating the document

To update document for history step application given a Document document, a session history entry entry, a boolean doNotReactivate, integers scriptHistoryLength and scriptHistoryIndex, NavigationType-or-null navigationType, an optional list of session history entries entriesForNavigationAPI, and an optional session history entry previousEntryForActivation:

Let documentIsNew be true if document's latest entry is null; otherwise false.
Let documentsEntryChanged be true if document's latest entry is not entry; otherwise false.
Set document's history object's index to scriptHistoryIndex.
Set document's history object's length to scriptHistoryLength.
Let navigation be history's relevant global object's navigation API.
If documentsEntryChanged is true, then:
1. Let oldURL be document's latest entry's URL.
2. Set document's latest entry to entry.
3. Restore the history object state given document and entry.
4. If documentIsNew is false, then:
  1. Assert: navigationType is not null.
  2. Update the navigation API entries for a same-document navigation given navigation, entry, and navigationType.
  3. Fire an event named popstate at document's relevant global object, using PopStateEvent, with the state attribute initialized to document's history object's state and hasUAVisualTransition initialized to true if a visual transition, to display a cached rendered state of the latest entry, was done by the user agent.
  4. Restore persisted state given entry.
  5. If oldURL's fragment is not equal to entry's URL's fragment, then queue a global task on the DOM manipulation task source given document's relevant global object to fire an event named hashchange at document's relevant global object, using HashChangeEvent, with the oldURL attribute initialized to the serialization of oldURL and the newURL attribute initialized to the serialization of entry's URL.
5. Otherwise:
  1. Assert: entriesForNavigationAPI is given.
  2. Restore persisted state given entry.
  3. Initialize the navigation API entries for a new document given navigation, entriesForNavigationAPI, and entry.
If all the following are true:
- previousEntryForActivation is given;
- navigationType is non-null; and
- navigationType is "reload" or previousEntryForActivation's document is not document,
then:
1. If navigation's activation is null, then set navigation's activation to a new NavigationActivation object in navigation's relevant realm.
2. Let previousEntryIndex be the result of getting the navigation API entry index of previousEntryForActivation within navigation.
3. If previousEntryIndex is non-negative, then set activation's old entry to navigation's entry list[previousEntryIndex].
4. Otherwise, if all the following are true:
  - navigationType is "replace";
  - previousEntryForActivation's document state's origin is same origin with document's origin; and
  - previousEntryForActivation's document's initial about:blank is false,
  then set activation's old entry to a new NavigationHistoryEntry in navigation's relevant realm, whose session history entry is previousEntryForActivation.
5. Set activation's new entry to navigation's current entry.
6. Set activation's navigation type to navigationType.
If documentIsNew is true, then:
1. Try to scroll to the fragment for document.
2. At this point scripts may run for the newly-created document document.
Otherwise, if documentsEntryChanged is false and doNotReactivate is false, then:
1. Assert: entriesForNavigationAPI is given.
2. Reactivate document given entry and entriesForNavigationAPI.
documentsEntryChanged can be false for one of two reasons: either we are restoring from bfcache, or we are asynchronously finishing up a synchronous navigation which already synchronously set document's latest entry. The doNotReactivate argument distinguishes between these two cases.

To restore the history object state given Document document and session history entry entry:

Let targetRealm be document's relevant realm.
Let state be StructuredDeserialize(entry's classic history API state, targetRealm). If this throws an exception, catch it and let state be null.
Set document's history object's state to state.

To make active a Document document:

Let window be document's relevant global object.
Set document's browsing context's WindowProxy's [[Window]] internal slot value to window.
Set document's visibility state to document's node navigable's traversable navigable's system visibility state.
Queue a new VisibilityStateEntry whose visibility state is document's visibility state and whose timestamp is zero.
Set window's relevant settings object's execution ready flag.

To reactivate a Document document given a session history entry reactivatedEntry and a list of session history entries entriesForNavigationAPI:

This algorithm updates document after it has come out of bfcache, i.e., after it has been made fully active again. Other specifications that want to watch for this change to the fully active state are encouraged to add steps into this algorithm, so that the ordering of events that happen in effect of the change is clear.

For each formControl of form controls in document with an autofill field name of "off", invoke the reset algorithm for formControl.
If document's suspended timer handles is not empty:
1. Assert: document's suspension time is not zero.
2. Let suspendDuration be the current high resolution time minus document's suspension time.
3. Let activeTimers be document's relevant global object's map of active timers.
4. For each handle in document's suspended timer handles, if activeTimers[handle] exists, then increase activeTimers[handle] by suspendDuration.
Update the navigation API entries for reactivation given document's relevant global object's navigation API, entriesForNavigationAPI, and reactivatedEntry.
If document's current document readiness is "complete", and document's page showing flag is false, then:
1. Set document's page showing flag to true.
2. Set document's has been revealed to false.
3. Update the visibility state of document to "visible".
4. Fire a page transition event named pageshow at document's relevant global object with true.

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

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.)
Queue a global task on the navigation and traversal 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 document.
3. If document's indicated part is still null, then try to scroll to the fragment for document.

To make document unsalvageable, given a Document document and a string reason:

Let details be a new not restored reason details whose reason is reason.
Append details to document's bfcache blocking details.
Set document's salvageable state to false.

To build not restored reasons for document state given Document document:

Let notRestoredReasonsForDocument be a new not restored reasons.
Set notRestoredReasonsForDocument's URL to document's URL.
If document's node navigable's container is an iframe element, then:
1. Set notRestoredReasonsForDocument's src to the value of document's node navigable's container's src attribute.
2. Set notRestoredReasonsForDocument's id to the value of document's node navigable's container's id attribute.
3. Set notRestoredReasonsForDocument's name to the value of document's node navigable's container's name attribute.
Set notRestoredReasonsForDocument's reasons to a clone of document's bfcache blocking details.
For each navigable of document's document-tree child navigables:
1. Let childDocument be navigable's active document.
2. Build not restored reasons for document state given childDocument.
3. Append childDocument's not restored reasons to notRestoredReasonsForDocument's children.
Set document's node navigable's active session history entry's document state's not restored reasons to notRestoredReasonsForDocument.

To build not restored reasons for a top-level traversable and its descendants given top-level traversable topLevelTraversable:

Build not restored reasons for document state given topLevelTraversable's active document.
Let crossOriginDescendants be an empty list.
For each childNavigable of topLevelTraversable's active document's descendant navigables:
1. If childNavigable's active document's origin is not same origin with topLevelTraversable's active document's origin, then append childNavigable to crossOriginDescendants.
Let crossOriginDescendantsPreventsBfcache be false.
For each crossOriginNavigable of crossOriginDescendants:
1. Let reasonsForCrossOriginChild be crossOriginNavigable's active document's document state's not restored reasons.
2. If reasonsForCrossOriginChild's reasons is not empty, set crossOriginDescendantsPreventsBfcache to true.
3. Set reasonsForCrossOriginChild's URL to null.
4. Set reasonsForCrossOriginChild's reasons to null.
5. Set reasonsForCrossOriginChild's children to null.
If crossOriginDescendantsPreventsBfcache is true, make document unsalvageable given topLevelTraversable's active document and "masked".

7.4.6.3 Revealing the document

A Document has a boolean has been revealed, initially false. It is used to ensure that the pagereveal event is fired once for each activation of the Document (once when it's rendered initially, and once for each reactivation).

To reveal a Document document:

If document's has been revealed is true, then return.
Set document's has been revealed to true.
Let transition be the result of resolving inbound cross-document view-transition for document.
Fire an event named pagereveal at document's relevant global object, using PageRevealEvent with its viewTransition set to transition.
If transition is not null, then:
1. Prepare to run script given document's relevant settings object.
2. Activate transition.
3. Clean up after running script given document's relevant settings object.
Activating a view transition might resolve/reject promises, so by wrapping the activation with prepare/cleanup we ensure those promises are handled before the next rendering step.

Though pagereveal is guaranteed to be fired during the first update the rendering step that displays an up-to-date version of the page, user agents are free to display a cached frame of the page before firing it. This prevents the presence of a pagereveal handler from delaying the presentation of such cached frame.

7.4.6.4 Scrolling to a fragment

To scroll to the fragment given a Document document:

If document's indicated part is null, then set document's target element to null.
Otherwise, if document's indicated part is top of the document, then:
1. Set document's target element to null.
2. Scroll to the beginning of the document for document. [CSSOMVIEW]
3. Return.
Otherwise:
1. Assert: document's indicated part is an element.
2. Let target be document's indicated part.
3. Set document's target element to target.
4. Run the ancestor details revealing algorithm on target.
5. Run the ancestor hidden-until-found revealing algorithm on target.
6. Scroll target into view, with behavior set to "auto", block set to "start", and inline set to "nearest". [CSSOMVIEW]
7. Run the focusing steps for target, with the Document's viewport as the fallback target.
8. Move the sequential focus navigation starting point to target.

A Document's indicated part is the one that its URL's fragment identifies, or null if the fragment does not identify anything. 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 an HTML document document, its indicated part is the result of selecting the indicated part given document and document's URL.

To select the indicated part given a Document document and a URL url:

If document's URL does not equal url with exclude fragments set to true, then return null.
Let fragment be url's fragment.
If fragment is the empty string, then return the special value top of the document.
Let potentialIndicatedElement be the result of finding a potential indicated element given document and fragment.
If potentialIndicatedElement is not null, then return potentialIndicatedElement.
Let fragmentBytes be the result of percent-decoding fragment.
Let decodedFragment be the result of running UTF-8 decode without BOM on fragmentBytes.
Set potentialIndicatedElement to the result of finding a potential indicated element given document and decodedFragment.
If potentialIndicatedElement is not null, then return potentialIndicatedElement.
If decodedFragment is an ASCII case-insensitive match for the string top, then return the top of the document.
Return null.

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

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

7.4.6.5 Persisted history entry state

To save persisted state to a session history entry entry:

Set the scroll position data of entry to contain the scroll positions for all of entry's document's restorable scrollable regions.
Optionally, update entry's persisted user state to reflect any state that the user agent wishes to persist, such as the values of form fields.

To restore persisted state from a session history entry entry:

If entry's scroll restoration mode is "auto", and entry's document's relevant global object's navigation API's suppress normal scroll restoration during ongoing navigation is false, then restore scroll position data given entry.

The user agent not restoring scroll positions does not imply that scroll positions 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.

If suppress normal scroll restoration during ongoing navigation is true, then restoring scroll position data might still happen at a later point, as part of finishing the relevant NavigateEvent, or via a navigateEvent.scroll() method call.
Optionally, update other aspects of entry's document and its rendering, for instance values of form fields, that the user agent had previously recorded in entry's persisted user state.

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

Restoring the value of form controls as part of this process does not fire any input or change events, but can trigger the formStateRestoreCallback of form-associated custom elements.

Each Document has a boolean has been scrolled by the user, initially false. If the user scrolls the document, the user agent must set that document's has been scrolled by the user to true.

The restorable scrollable regions of a Document document are document's viewport, and all of document's scrollable regions excepting any navigable containers.

Child navigable scroll restoration is handled as part of state restoration for the session history entry for those navigables' Documents.

To restore scroll position data given a session history entry entry:

Let document be entry's document.
If document's has been scrolled by the user is true, then the user agent should return.
The user agent should attempt to use entry's scroll position data to restore the scroll positions of entry's document's restorable scrollable regions. The user agent may continue to attempt to do so periodically, until document's has been scrolled by the user becomes true.

This is formulated as an attempt, which is potentially repeated until success or until the user scrolls, due to the fact that relevant content indicated by the scroll position data might take some time to load from the network.

Scroll restoration might be affected by scroll anchoring. [CSSSCROLLANCHORING]

HTML: The Living Standard

7.4 Navigation and session history

7.4.1 Session history

7.4.1.1 Session history entries

7.4.1.2 Document state

7.4.1.3 Centralized modifications of session history

7.4.1.4 Low-level operations on session history

7.4.2 Navigation

7.4.2.1 Supporting concepts

7.4.2.2 Beginning navigation

7.4.2.3 Ending navigation

7.4.2.3.1 The usual cross-document navigation case

7.4.2.3.2 The `javascript:` URL special case

7.4.2.3.3 Fragment navigations

7.4.2.3.4 Non-fetch schemes and external software

7.4.2.4 Preventing navigation

7.4.2.5 Aborting navigation

7.4.3 Reloading and traversing

7.4.4 Non-fragment synchronous "navigations"

7.4.5 Populating a session history entry

7.4.6 Applying the history step

7.4.6.1 Updating the traversable

7.4.6.2 Updating the document

7.4.6.3 Revealing the document

7.4.6.4 Scrolling to a fragment

7.4.6.5 Persisted history entry state

7.4 Navigation and session history

7.4.1 Session history

7.4.1.1 Session history entries

7.4.1.2 Document state

7.4.1.3 Centralized modifications of session history

7.4.1.4 Low-level operations on session history

7.4.2 Navigation

7.4.2.1 Supporting concepts

7.4.2.2 Beginning navigation

7.4.2.3 Ending navigation

7.4.2.3.1 The usual cross-document navigation case

7.4.2.3.2 The javascript: URL special case

7.4.2.3.3 Fragment navigations

7.4.2.3.4 Non-fetch schemes and external software

7.4.2.4 Preventing navigation

7.4.2.5 Aborting navigation

7.4.3 Reloading and traversing

7.4.4 Non-fragment synchronous "navigations"

7.4.5 Populating a session history entry

7.4.6 Applying the history step

7.4.6.1 Updating the traversable

7.4.6.2 Updating the document

7.4.6.3 Revealing the document

7.4.6.4 Scrolling to a fragment

7.4.6.5 Persisted history entry state

7.4.2.3.2 The `javascript:` URL special case