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HTML Standard

Living Standard — Last Updated 9 May 2025

1. 1. 7.3 Infrastructure for sequences of documents
      1. 7.3.1 Navigables
         1. 7.3.1.1 Traversable navigables
         2. 7.3.1.2 Top-level traversables
         3. 7.3.1.3 Child navigables
         4. 7.3.1.4 Jake diagrams
         5. 7.3.1.5 Related navigable collections
         6. 7.3.1.6 Navigable destruction
         7. 7.3.1.7 Navigable target names
      2. 7.3.2 Browsing contexts
         1. 7.3.2.1 Creating browsing contexts
         2. 7.3.2.2 Related browsing contexts
         3. 7.3.2.3 Groupings of browsing contexts
      3. 7.3.3 Fully active documents

This standard contains several related concepts for grouping sequences of documents. As a brief, non-normative summary:

• Navigables are a user-facing representation of a sequence of documents, i.e., they represent something that can be navigated between documents. Typical examples are tabs or windows in a web browser, or iframes, or frames in a frameset.

• Traversable navigables are a special type of navigable which control the session history of themselves and of their descendant navigables. That is, in addition to their own series of documents, they represent a tree of further series of documents, plus the ability to linearly traverse back and forward through a flattened view of this tree.

• Browsing contexts are a developer-facing representation of a series of documents. They correspond 1:1 with WindowProxy objects. Each navigable can present a series of browsing contexts, with switches between those browsing contexts occuring under certain well-defined circumstances.

Most of this standard works in the language of navigables, but certain APIs expose the existence of browsing context switches, and so some parts of the standard need to work in terms of browsing contexts.

A navigable presents a Document to the user via its active session history entry. Each navigable has:

• An id, a new unique internal value.

• A parent, a navigable or null.

• A current session history entry, a session history entry.

  This can only be modified within the session history traversal queue of the parent traversable navigable.

• An active session history entry, a session history entry.

  This can only be modified from the event loop of the active session history entry's document.

• An is closing boolean, initially false.

  This is only ever set to true for top-level traversable navigables.

• An is delaying load events boolean, initially false.

  This is only ever set to true in cases where the navigable's parent is non-null.

The current session history entry and the active session history entry are usually the same, but they get out of sync when:

• Synchronous navigations are performed. This causes the active session history entry to temporarily step ahead of the current session history entry.

• A non-displayable, non-error response is received when applying the history step. This updates the current session history entry but leaves the active session history entry as-is.

A navigable's active document is its active session history entry's document.

This can be safely read from within the session history traversal queue of the navigable's top-level traversable. Although a navigable's active history entry can change synchronously, the new entry will always have the same Document.

A navigable's active browsing context is its active document's browsing context. If this navigable is a traversable navigable, then its active browsing context will be a top-level browsing context.

A navigable's active WindowProxy is its active browsing context's associated WindowProxy.

A navigable's active window is its active WindowProxy's [[Window]].

This will always equal the navigable's active document's relevant global object; this is kept in sync by the make active algorithm.

A navigable's target name is its active session history entry's document state's navigable target name.

To get the node navigable of a node node, return the navigable whose active document is node's node document, or null if there is no such navigable.

To initialize the navigable navigable navigable, given a document state documentState and an optional navigable-or-null parent (default null):

1. Assert: documentState's document is non-null.

2. Let entry be a new session history entry, with

   URL

   documentState's document's URL

   document state

   documentState

   The caller of this algorithm is responsible for initializing entry's step; it will be left as "pending" until that is complete.

3. Set navigable's current session history entry to entry.

4. Set navigable's active session history entry to entry.

5. Set navigable's parent to parent.

A traversable navigable is a navigable that also controls which session history entry should be the current session history entry and active session history entry for itself and its descendant navigables.

In addition to the properties of a navigable, a traversable navigable has:

• A current session history step, a number, initially 0.

• Session history entries, a list of session history entries, initially a new list.

• A session history traversal queue, a session history traversal parallel queue, the result of starting a new session history traversal parallel queue.

• A running nested apply history step boolean, initially false.

• A system visibility state, which is either "hidden" or "visible".

  See the page visibility section for the requirements on this item.

• An is created by web content boolean, initially false.

To get the traversable navigable of a navigable inputNavigable:

1. Let navigable be inputNavigable.

2. While navigable is not a traversable navigable, set navigable to navigable's parent.

3. Return navigable.

A top-level traversable is a traversable navigable with a null parent.

Currently, all traversable navigables are top-level traversables. Future proposals envision introducing non-top-level traversables.

A user agent holds a top-level traversable set (a set of top-level traversables). These are typically presented to the user in the form of browser windows or browser tabs.

To get the top-level traversable of a navigable inputNavigable:

1. Let navigable be inputNavigable.

2. While navigable's parent is not null, set navigable to navigable's parent.

3. Return navigable.

To create a new top-level traversable given a browsing context-or-null opener, a string targetName, and an optional navigable openerNavigableForWebDriver:

1. Let document be null.

2. If opener is null, then set document to the second return value of creating a new top-level browsing context and document.

3. Otherwise, set document to the second return value of creating a new auxiliary browsing context and document given opener.

4. Let documentState be a new document state, with

   document

   document

   initiator origin

   null if opener is null; otherwise, document's origin

   origin

   document's origin

   navigable target name

   targetName

   about base URL

   document's about base URL

5. Let traversable be a new traversable navigable.

6. Initialize the navigable traversable given documentState.

7. Let initialHistoryEntry be traversable's active session history entry.

8. Set initialHistoryEntry's step to 0.

9. Append initialHistoryEntry to traversable's session history entries.

10. If opener is non-null, then legacy-clone a traversable storage shed given opener's top-level traversable and traversable. [STORAGE]

11. Append traversable to the user agent's top-level traversable set.

12. Invoke WebDriver BiDi navigable created with traversable and openerNavigableForWebDriver.

13. Return traversable.

To create a fresh top-level traversable given a URL initialNavigationURL and an optional POST resource-or-null initialNavigationPostResource (default null):

1. Let traversable be the result of creating a new top-level traversable given null and the empty string.

2. Navigate traversable to initialNavigationURL using traversable's active document, with documentResource set to initialNavigationPostResource.

   We treat these initial navigations as traversable navigating itself, which will ensure all relevant security checks pass.

3. Return traversable.

Certain elements (for example, iframe elements) can present a navigable to the user. These elements are called navigable containers.

Each navigable container has a content navigable, which is either a navigable or null. It is initially null.

The container of a navigable navigable is the navigable container whose content navigable is navigable, or null if there is no such element.

The container document of a navigable navigable is the result of running these steps:

1. If navigable's container is null, then return null.

2. Return navigable's container's node document.

   This is equal to navigable's container's shadow-including root as navigable's container has to be connected.

The container document of a Document document is the result of running these steps:

1. If document's node navigable is null, then return null.

2. Return document's node navigable's container document.

A navigable navigable is a child navigable of another navigable potentialParent when navigable's parent is potentialParent. We can also just say that a navigable "is a child navigable", which means that its parent is non-null.

All child navigables are the content navigable of their container.

The content document of a navigable container container is the result of running these steps:

1. If container's content navigable is null, then return null.

2. Let document be container's content navigable's active document.

3. If document's origin and container's node document's origin are not same origin-domain, then return null.

4. Return document.

The content window of a navigable container container is the result of running these steps:

1. If container's content navigable is null, then return null.

2. Return container's content navigable's active WindowProxy's object.

To create a new child navigable, given an element element:

1. Let parentNavigable be element's node navigable.

2. Let group be element's node document's browsing context's top-level browsing context's group.

3. Let browsingContext and document be the result of creating a new browsing context and document given element's node document, element, and group.

4. Let targetName be null.

5. If element has a name content attribute, then set targetName to the value of that attribute.

6. Let documentState be a new document state, with

   document

   document

   initiator origin

   document's origin

   origin

   document's origin

   navigable target name

   targetName

   about base URL

   document's about base URL

7. Let navigable be a new navigable.

8. Initialize the navigable navigable given documentState and parentNavigable.

9. Set element's content navigable to navigable.

10. Let historyEntry be navigable's active session history entry.

11. Let traversable be parentNavigable's traversable navigable.

12. Append the following session history traversal steps to traversable:

    1. Let parentDocState be parentNavigable's active session history entry's document state.

    2. Let parentNavigableEntries be the result of getting session history entries for parentNavigable.

    3. Let targetStepSHE be the first session history entry in parentNavigableEntries whose document state equals parentDocState.

    4. Set historyEntry's step to targetStepSHE's step.

    5. Let nestedHistory be a new nested history whose id is navigable's id and entries list is « historyEntry ».

    6. Append nestedHistory to parentDocState's nested histories.

    7. Update for navigable creation/destruction given traversable.

13. Invoke WebDriver BiDi navigable created with traversable.

A useful method for visualizing sequences of documents, and in particular navigables and their session history entries, is the Jake diagram. A typical Jake diagram is the following:

Here, each numbered column denotes a possible value for the traversable's session history step. Each labeled row depicts a navigable, as it transitions between different URLs and documents. The first, labeled top, being the top-level traversable, and the others being child navigables. The documents are given by the background color of each cell, with a new background color indicating a new document in that navigable. The URLs are given by the text content of the cells; usually they are given as relative URLs for brevity, unless a cross-origin case is specifically under investigation. A given navigable might not exist at a given step, in which case the corresponding cells are empty. The bold-italic step number depicts the current session history step of the traversable, and all cells with bold-italic URLs represent the current session history entry for that row's navigable.

Thus, the above Jake diagram depicts the following sequence of events:

1. A top-level traversable is created, starting a the URL /t-a, with two child navigables starting at /i-0-a and /i-1-a respectively.

2. The first child navigable is navigated to another document, with URL /i-0-b.

3. The second child navigable is navigated to another document, with URL /i-1-b.

4. The top-level traversable is navigated to the same document, updating its URL to /t-a#foo.

5. The top-level traversable is navigated to another document, with URL /t-b. (Notice how this document, of course, does not carry over the old document's child navigables.)

6. The traversable was traversed by a delta of −3, back to step 1.

Jake diagrams are a powerful tool for visualizing the interactions of multiple navigables, navigations, and traversals. They cannot capture every possible interaction — for example, they only work with a single level of nesting — but we will have ocassion to use them to illustrate several complex situations throughout this standard.

Jake diagrams are named after their creator, the inimitable Jake Archibald.

It is often helpful in this standard's algorithms to look at collections of navigables starting at a given . This section contains a curated set of algorithms for collecting those navigables.

The return values of these algorithms are ordered so that parents appears before their children. Callers rely on this ordering.

Starting with a , rather than a navigable, is generally better because it makes the caller cognizant of whether they are starting with a fully active or not. Although non-fully active s do have ancestor and descendant navigables, they often behave as if they don't (e.g., in the getter).

The ancestor navigables of a document are given by these steps:

1. Let navigable be document's node navigable's parent.

2. Let ancestors be an empty list.

3. While navigable is not null:

   1. Prepend navigable to ancestors.

   2. Set navigable to navigable's parent.

4. Return ancestors.

The inclusive ancestor navigables of a document are given by these steps:

1. Let navigables be document's ancestor navigables.

2. Append document's node navigable to navigables.

3. Return navigables.

The descendant navigables of a document are given by these steps:

1. Let navigables be new list.

2. Let navigableContainers be a list of all shadow-including descendants of document that are navigable containers, in shadow-including tree order.

3. For each navigableContainer of navigableContainers:

   1. If navigableContainer's content navigable is null, then continue.

   2. Extend navigables with navigableContainer's content navigable's active document's inclusive descendant navigables.

4. Return navigables.

The inclusive descendant navigables of a document are given by these steps:

1. Let navigables be « document's node navigable ».

2. Extend navigables with document's descendant navigables.

3. Return navigables.

These descendant-collecting algorithms are described as looking at the DOM tree of descendant objects. In reality, this is often not feasible since the DOM tree can be in another process from the caller of the algorithm. Instead, implementations generally replicate the appropriate trees across processes.

The document-tree child navigables of a document are given by these steps:

1. If document's node navigable is null, then return the empty list.

2. Let navigables be new list.

3. Let navigableContainers be a list of all descendants of document that are navigable containers, in tree order.

4. For each navigableContainer of navigableContainers:

   1. If navigableContainer's content navigable is null, then continue.

   2. Append navigableContainer's content navigable to navigables.

5. Return navigables.

To destroy a child navigable given a navigable container container:

1. Let navigable be container's content navigable.

2. If navigable is null, then return.

3. Set container's content navigable to null.

4. Inform the navigation API about child navigable destruction given navigable.

5. Destroy a document and its descendants given navigable's active document.

6. Let parentDocState be container's node navigable's active session history entry's document state.

7. Remove the nested history from parentDocState's nested histories whose id equals navigable's id.

8. Let traversable be container's node navigable's traversable navigable.

9. Append the following session history traversal steps to traversable:

   1. Update for navigable creation/destruction given traversable.

10. Invoke WebDriver BiDi navigable destroyed with navigable.

To destroy a top-level traversable traversable:

1. Let browsingContext be traversable's active browsing context.

2. For each historyEntry in traversable's session history entries in what order?:

   1. Let document be historyEntry's document.

   2. If document is not null, then destroy a document and its descendants given document.

3. Remove browsingContext.

4. Remove traversable from the user interface (e.g., close or hide its tab in a tabbed browser).

5. Remove traversable from the user agent's top-level traversable set.

6. Invoke WebDriver BiDi navigable destroyed with traversable.

User agents may destroy a top-level traversable at any time (typically, in response to user requests).

To close a top-level traversable traversable:

1. If traversable's is closing is true, then return.

2. Definitely close traversable.

To definitely close a top-level traversable traversable:

1. Let toUnload be traversable's active document's inclusive descendant navigables.

2. If the result of checking if unloading is canceled for toUnload is true, then return.

3. Append the following session history traversal steps to traversable:

   1. Let afterAllUnloads be an algorithm step which destroys traversable.

   2. Unload a document and its descendants given traversable's active document, null, and afterAllUnloads.

The close vs. definitely close separation allows other specifications to call close and have it be a no-op if the top-level traversable is already closing due to JavaScript code calling window.close().

Navigables can be given target names, which are strings allowing certain APIs (such as window.open() or the a element's target attribute) to target navigations at that navigable.

A valid navigable target name is any string with at least one character that does not contain both an ASCII tab or newline and a U+003C (<), and it does not start with a U+005F (_). (Names starting with a U+005F (_) are reserved for special keywords.)

A valid navigable target name or keyword is any string that is either a valid navigable target name or that is an ASCII case-insensitive match for one of: _blank, _self, _parent, or _top.

These values have different meanings based on whether the page is sandboxed or not, as summarized in the following (non-normative) table. In this table, "current" means the navigable that the link or script is in, "parent" means the parent of the navigable that the link or script is in, "top" means the top-level traversable of the navigable that the link or script is in, "new" means a new traversable navigable with a null parent (which may use an auxiliary browsing context, subject to various user preferences and user agent policies), "none" means that nothing will happen, and "maybe new" means the same as "new" if the "allow-popups" keyword is also specified on the sandbox attribute (or if the user overrode the sandboxing), and the same as "none" otherwise.

Most of the restrictions on sandboxed browsing contexts are applied by other algorithms, e.g. the navigation algorithm, not the rules for choosing a navigable given below.

To find a navigable by target name given a string name and a navigable currentNavigable:

1. Let currentDocument be currentNavigable's active document.

2. Let sourceSnapshotParams be the result of snapshotting source snapshot params given currentDocument.

3. Let subtreesToSearch be an implementation-defined choice of one of the following:

   • « currentNavigable's traversable navigable, currentNavigable »

   • the inclusive ancestor navigables of currentDocument

   Issue #10848 tracks settling on one of these two possibilities, to achieve interoperability.

4. For each subtreeToSearch of subtreesToSearch, in reverse order:

   1. Let documentToSearch be subtreeToSearch's active document.

   2. For each navigable of the inclusive descendant navigables of documentToSearch:

      1. If currentNavigable is not allowed by sandboxing to navigate navigable given sourceSnapshotParams, then optionally continue.

         Issue #10849 tracks making this check required, to achieve interoperability.

      2. If navigable's target name is name, then return navigable.

5. Let currentTopLevelBrowsingContext be currentNavigable's active browsing context's top-level browsing context.

6. Let group be currentTopLevelBrowsingContext's group.

7. For each topLevelBrowsingContext of group's browsing context set, in an implementation-defined order (the user agent should pick a consistent ordering, such as the most recently opened, most recently focused, or more closely related):

   Issue #10850 tracks picking a specific ordering, to achieve interoperability.

   1. If currentTopLevelBrowsingContext is topLevelBrowsingContext, then continue.

   2. Let documentToSearch be topLevelBrowsingContext's active document.

   3. For each navigable of the inclusive descendant navigables of documentToSearch:

      1. If currentNavigable's active browsing context is not familiar with navigable's active browsing context, then continue.

      2. If currentNavigable is not allowed by sandboxing to navigate navigable given sourceSnapshotParams, then optionally continue.

         Issue #10849 tracks making this check required, to achieve interoperability.

      3. If navigable's target name is name, then return navigable.

8. Return null.

The rules for choosing a navigable, given a string name, a navigable currentNavigable, and a boolean noopener are as follows:

1. Let chosen be null.

2. Let windowType be "existing or none".

3. Let sandboxingFlagSet be currentNavigable's active document's active sandboxing flag set.

4. If name is the empty string or an ASCII case-insensitive match for "_self", then set chosen to currentNavigable.

5. Otherwise, if name is an ASCII case-insensitive match for "_parent", set chosen to currentNavigable's parent, if any, and currentNavigable otherwise.

6. Otherwise, if name is an ASCII case-insensitive match for "_top", set chosen to currentNavigable's traversable navigable.

7. Otherwise, if name is not an ASCII case-insensitive match for "_blank" and noopener is false, then set chosen to the result of finding a navigable by target name given name and currentNavigable.

8. If chosen is null, then a new top-level traversable is being requested, and what happens depends on the user agent's configuration and abilities — it is determined by the rules given for the first applicable option from the following list:

   The user agent may inform the user that a popup has been blocked.

   If sandboxingFlagSet has the sandboxed auxiliary navigation browsing context flag set

   The user agent may report to a developer console that a popup has been blocked.

   If the user agent has been configured such that in this instance it will create a new top-level traversable

   1. Consume user activation of currentNavigable's active window.

   2. Set windowType to "new and unrestricted".

   3. Let currentDocument be currentNavigable's active document.

   4. If currentDocument's opener policy's value is "same-origin" or "same-origin-plus-COEP", and currentDocument's origin is not same origin with currentDocument's relevant settings object's top-level origin, then:

      1. Set noopener to true.

      2. Set name to "_blank".

      3. Set windowType to "new with no opener".

      In the presence of an opener policy, nested documents that are cross-origin with their top-level browsing context's active document always set noopener to true.

   5. Let targetName be the empty string.

   6. If name is not an ASCII case-insensitive match for "_blank", then set targetName to name.

   7. If noopener is true, then set chosen to the result of creating a new top-level traversable given null, targetName, and currentNavigable.

   8. Otherwise:

      1. Set chosen to the result of creating a new top-level traversable given currentNavigable's active browsing context, targetName, and currentNavigable.

      2. If sandboxingFlagSet's sandboxed navigation browsing context flag is set, then set chosen's active browsing context's one permitted sandboxed navigator to currentNavigable's active browsing context.

   9. If sandboxingFlagSet's sandbox propagates to auxiliary browsing contexts flag is set, then all the flags that are set in sandboxingFlagSet must be set in chosen's active browsing context's popup sandboxing flag set.

   10. Set chosen's is created by web content to true.

   If the newly created navigable chosen is immediately navigated, then the navigation will be done as a "replace" navigation.

   If the user agent has been configured such that in this instance it will choose currentNavigable

   Set chosen to currentNavigable.

   If the user agent has been configured such that in this instance it will not find a navigable

   Do nothing.

   User agents are encouraged to provide a way for users to configure the user agent to always choose currentNavigable.

9. Return chosen and windowType.

A browsing context is a programmatic representation of a series of documents, multiple of which can live within a single navigable. Each browsing context has a corresponding WindowProxy object, as well as the following:

• An opener browsing context, a browsing context or null, initially null.

• An opener origin at creation, an origin or null, initially null.

• An boolean, initially false.

  The only mandatory impact in this specification of is popup is on the visible getter of the relevant BarProp objects. However, user agents might also use it for user interface considerations.

• An is auxiliary boolean, initially false.

• An initial URL, a URL or null, initially null.

• A virtual browsing context group ID integer, initially 0. This is used by opener policy reporting, to keep track of the browsing context group switches that would have happened if the report-only policy had been enforced.

A browsing context's active window is its WindowProxy object's [[Window]] internal slot value. A browsing context's active document is its active window's associated Document.

A browsing context's top-level traversable is its active document's node navigable's top-level traversable.

A browsing context whose is auxiliary is true is known as an auxiliary browsing context. Auxiliary browsing contexts are always top-level browsing contexts.

It's unclear whether a separate is auxiliary concept is necessary. In issue #5680, it is indicated that we may be able to simplify this by using whether or not the opener browsing context is null.

Modern specifications should avoid using the browsing context concept in most cases, unless they are dealing with the subtleties of browsing context group switches and agent cluster allocation. Instead, the Document and navigable concepts are usually more appropriate.

A Document's browsing context is a browsing context or null, initially null.

A Document does not necessarily have a non-null browsing context. In particular, data mining tools are likely to never instantiate browsing contexts. A Document created using an API such as createDocument() never has a non-null browsing context. And the Document originally created for an iframe element, which has since been removed from the document, has no associated browsing context, since that browsing context was nulled out.

In general, there is a 1-to-1 mapping from the Window object to the Document object, as long as the Document object has a non-null browsing context. There is one exception. A Window can be reused for the presentation of a second Document in the same browsing context, such that the mapping is then 1-to-2. This occurs when a browsing context is navigated from the initial about:blank Document to another, which will be done with replacement.

To create a new browsing context and document, given null or a Document object creator, null or an element embedder, and a browsing context group group:

1. Let browsingContext be a new browsing context.

2. Let unsafeContextCreationTime be the .

3. Let creatorOrigin be null.

4. Let creatorBaseURL be null.

5. If creator is non-null, then:

   1. Set creatorOrigin to creator's origin.

   2. Set creatorBaseURL to creator's document base URL.

   3. Set browsingContext's virtual browsing context group ID to creator's browsing context's top-level browsing context's virtual browsing context group ID.

6. Let sandboxFlags be the result of determining the creation sandboxing flags given browsingContext and embedder.

7. Let origin be the result of determining the origin given about:blank, sandboxFlags, and creatorOrigin.

8. Let permissionsPolicy be the result of creating a permissions policy given embedder and origin. [PERMISSIONSPOLICY]

9. Let agent be the result of obtaining a similar-origin window agent given origin, group, and false.

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

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

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

11. Let topLevelCreationURL be about:blank if embedder is null; otherwise embedder's relevant settings object's top-level creation URL.

12. Let topLevelOrigin be origin if embedder is null; otherwise embedder's relevant settings object's top-level origin.

13. Set up a window environment settings object with about:blank, realm execution context, null, topLevelCreationURL, and topLevelOrigin.

14. Let loadTimingInfo be a new document load timing info with its navigation start time set to the result of calling coarsen time with unsafeContextCreationTime and the new environment settings object's cross-origin isolated capability.

15. Let document be a new Document, with:

    type

    "html"

    content type

    "text/html"

    mode

    "quirks"

    origin

    origin

    browsing context

    browsingContext

    permissions policy

    permissionsPolicy

    active sandboxing flag set

    sandboxFlags

    load timing info

    loadTimingInfo

    is initial about:blank

    true

    about base URL

    creatorBaseURL

    allow declarative shadow roots

    true

    custom element registry

    A new CustomElementRegistry object.

16. If creator is non-null, then:

    1. Set document's referrer to the serialization of creator's URL.

    2. Set document's policy container to a clone of creator's policy container.

    3. If creator's origin is same origin with creator's relevant settings object's top-level origin, then set document's opener policy to creator's browsing context's top-level browsing context's active document's opener policy.

17. Assert: document's URL and document's relevant settings object's creation URL are about:blank.

18. Mark document as ready for post-load tasks.

19. Populate with html/head/body given document.

20. Make active document.

21. Completely finish loading document.

22. Return browsingContext and document.

To create a new top-level browsing context and document:

1. Let group and document be the result of creating a new browsing context group and document.

2. Return group's browsing context set[0] and document.

To create a new auxiliary browsing context and document, given a browsing context opener:

1. Let openerTopLevelBrowsingContext be opener's top-level traversable's active browsing context.

2. Let group be openerTopLevelBrowsingContext's group.

3. Assert: group is non-null, as navigating invokes this directly.

4. Let browsingContext and document be the result of creating a new browsing context and document with opener's active document, null, and group.

5. Set browsingContext's is auxiliary to true.

6. Append browsingContext to group.

7. Set browsingContext's opener browsing context to opener.

8. Set browsingContext's virtual browsing context group ID to openerTopLevelBrowsingContext's virtual browsing context group ID.

9. Set browsingContext's opener origin at creation to opener's active document's origin.

10. Return browsingContext and document.

To determine the origin, given a URL url, a sandboxing flag set sandboxFlags, and an origin-or-null sourceOrigin:

1. If sandboxFlags has its sandboxed origin browsing context flag set, then return a new opaque origin.

2. If url is null, then return a new opaque origin.

3. If url is about:srcdoc, then:

   1. Assert: sourceOrigin is non-null.

   2. Return sourceOrigin.

4. If url matches about:blank and sourceOrigin is non-null, then return sourceOrigin.

5. Return url's origin.

The cases that return sourceOrigin result in two Documents that end up with the same underlying origin, meaning that document.domain affects both.

A browsing context potentialDescendant is said to be an ancestor of a browsing context potentialAncestor if the following algorithm returns true:

1. Let potentialDescendantDocument be potentialDescendant's active document.

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

3. Let ancestorBCs be the list obtained by taking the browsing context of the active document of each member of potentialDescendantDocument's ancestor navigables.

4. If ancestorBCs contains potentialAncestor, then return true.

5. Return false.

A top-level browsing context is a browsing context whose active document's node navigable is a traversable navigable.

It is not required to be a top-level traversable.

The top-level browsing context of a browsing context start is the result of the following algorithm:

1. If start's active document is not fully active, then return null.

2. Let navigable be start's active document's node navigable.

3. While navigable's parent is not null, set navigable to navigable's parent.

4. Return navigable's active browsing context.

A browsing context A is familiar with a second browsing context B if the following algorithm returns true:

1. If A's active document's origin is same origin with B's active document's origin, then return true.

2. If A's top-level browsing context is B, then return true.

3. If B is an auxiliary browsing context and A is familiar with B's opener browsing context, then return true.

4. If there exists an ancestor browsing context of B whose active document has the same origin as the active document of A, then return true.

   This includes the case where A is an ancestor browsing context of B.

5. Return false.

A top-level browsing context has an associated group (null or a browsing context group). It is initially null.

A user agent holds a browsing context group set (a set of browsing context groups).

A browsing context group holds a browsing context set (a set of top-level browsing contexts).

A top-level browsing context is added to the group when the group is created. All subsequent top-level browsing contexts added to the group will be auxiliary browsing contexts.

A browsing context group has an associated agent cluster map (a weak map of agent cluster keys to agent clusters). User agents are responsible for collecting agent clusters when it is deemed that nothing can access them anymore.

A browsing context group has an associated historical agent cluster key map, which is a map of origins to agent cluster keys. This map is used to ensure the consistency of the origin-keyed agent clusters feature by recording what agent cluster keys were previously used for a given origin.

The historical agent cluster key map only ever gains entries over the lifetime of the browsing context group.

A browsing context group has a cross-origin isolation mode, which is a cross-origin isolation mode. It is initially "none".

A cross-origin isolation mode is one of three possible values: "none", "logical", or "concrete".

To create a new browsing context group and document:

1. Let group be a new browsing context group.

2. Append group to the user agent's browsing context group set.

3. Let browsingContext and document be the result of creating a new browsing context and document with null, null, and group.

4. Append browsingContext to group.

5. Return group and document.

To append a top-level browsing context browsingContext to a browsing context group group:

1. Append browsingContext to group's browsing context set.

2. Set browsingContext's group to group.

To remove a top-level browsing context browsingContext:

1. Assert: browsingContext's group is non-null.

2. Let group be browsingContext's group.

3. Set browsingContext's group to null.

4. Remove browsingContext from group's browsing context set.

5. If group's browsing context set is empty, then remove group from the user agent's browsing context group set.

Append and remove are primitive operations that help define the lifetime of a browsing context group. They are called by higher-level creation and destruction operations for Documents and browsing contexts.

When there are no Document objects whose browsing context equals a given browsing context (i.e., all such Documents have been destroyed), and that browsing context's WindowProxy is eligible for garbage collection, then the browsing context will never be accessed again. If it is a top-level browsing context, then at this point the user agent must remove it.

A Document d is said to be fully active when d is the active document of a navigable navigable, and either navigable is a top-level traversable or navigable's container document is fully active.

Because they are associated with an element, child navigables are always tied to a specific Document, their container document, in their parent navigable. User agents must not allow the user to interact with child navigables whose container documents are not themselves fully active.

The following example illustrates how a Document can be the active document of its node navigable, while not being fully active. Here a.html is loaded into a browser window, b-1.html starts out loaded into an iframe as shown, and b-2.html and c.html are omitted (they can simply be an empty document).

<!-- a.html -->
<!DOCTYPE html>
<html lang="en">
<title>Navigable A</title>

<iframe src="b-1.html"></iframe>
<button onclick="frames[0].location.href = 'b-2.html'">Click me</button>

<!-- b-1.html -->
<!DOCTYPE html>
<html lang="en">
<title>Navigable B</title>

<iframe src="c.html"></iframe>

At this point, the documents given by a.html, b-1.html, and c.html are all the active documents of their respective node navigables. They are also all fully active.

After clicking on the button, and thus loading a new Document from b-2.html into navigable B, we have the following results:

• The a.html Document remains both the active document of navigable A, and fully active.

• The b-1.html Document is now not the active document of navigable B. As such it is also not fully active.

• The new b-2.html Document is now the active document of navigable B, and is also fully active.

• The c.html Document is still the active document of navigable C. However, since C's container document is the b-1.html Document, which is itself not fully active, this means the c.html Document is now not fully active.

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