7 Loading Web pages

7.1 Browsing contexts

A browsing context is an environment in which Document objects are presented to the user.

A tab or window in a Web browser typically contains a browsing context, as does an iframe.

Each browsing context has a corresponding WindowProxy object.

A browsing context has a session history, which lists the Document objects that that browsing context has presented, is presenting, or will present. At any time, one Document in each browsing context is designated the active document. A Document's browsing context is that browsing context whose session history contains the Document, if any. (A Document created using an API such as createDocument() has no browsing context.)

Each Document in a browsing context is associated with a Window object. A browsing context's WindowProxy object forwards everything to the browsing context's active document's Window object.

In general, there is a 1-to-1 mapping from the Window object to the Document object. There are two exceptions. First, 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, with replacement enabled. Second, a Document can end up being reused for several Window objects when the document.open() method is used, such that the mapping is then many-to-1.

A Document does not necessarily have a browsing context associated with it. In particular, data mining tools are likely to never instantiate browsing contexts.


A browsing context can have a creator browsing context, the browsing context that was responsible for its creation. If a browsing context has a parent browsing context, then that is its creator browsing context. Otherwise, if the browsing context has an opener browsing context, then that is its creator browsing context. Otherwise, the browsing context has no creator browsing context.

If a browsing context A has a creator browsing context, then the Document that was the active document of that creator browsing context at the time A was created is the creator Document.

7.1.1 Nested browsing contexts

Certain elements (for example, iframe elements) can instantiate further browsing contexts. These are called nested browsing contexts. If a browsing context P has a Document D with an element E that nests another browsing context C inside it, then C is said to be nested through D, and E is said to be the browsing context container of C. If the browsing context container element E is in the Document D, then P is said to be the parent browsing context of C and C is said to be a child browsing context of P. Otherwise, the nested browsing context C has no parent browsing context.

A browsing context A is said to be an ancestor of a browsing context B if there exists a browsing context A' that is a child browsing context of A and that is itself an ancestor of B, or if the browsing context A is the parent browsing context of B.

A browsing context that is not a nested browsing context has no parent browsing context, and is the top-level browsing context of all the browsing contexts for which it is an ancestor browsing context.

The transitive closure of parent browsing contexts for a nested browsing context gives the list of ancestor browsing contexts.

The list of the descendant browsing contexts of a Document d is the (ordered) list returned by the following algorithm:

  1. Let list be an empty list.

  2. For each child browsing context of d that is nested through an element that is in the Document d, in the tree order of the elements nesting those browsing contexts, run these substeps:

    1. Append that child browsing context to the list list.

    2. Append the list of the descendant browsing contexts of the active document of that child browsing context to the list list.

  3. Return the constructed list.

A Document is said to be fully active when it is the active document of its browsing context, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the Document through which it is nested is itself fully active.

Because they are nested through an element, child browsing contexts are always tied to a specific Document in their parent browsing context. User agents must not allow the user to interact with child browsing contexts of elements that are in Documents that are not themselves fully active.

A nested browsing context can have a seamless browsing context flag set, if it is embedded through an iframe element with a seamless attribute.

A nested browsing context can be put into a delaying load events mode. This is used when it is navigated, to delay the load event of the browsing context container before the new Document is created.

The document family of a browsing context consists of the union of all the Document objects in that browsing context's session history and the document families of all those Document objects. The document family of a Document object consists of the union of all the document families of the browsing contexts that are nested through the Document object.

window . top

Returns the WindowProxy for the top-level browsing context.

window . parent

Returns the WindowProxy for the parent browsing context.

window . frameElement

Returns the Element for the browsing context container.

Returns null if there isn't one.

Throws a SecurityError exception in cross-origin situations.

7.1.2 Auxiliary browsing contexts

It is possible to create new browsing contexts that are related to a top-level browsing context without being nested through an element. Such browsing contexts are called auxiliary browsing contexts. Auxiliary browsing contexts are always top-level browsing contexts.

An auxiliary browsing context has an opener browsing context, which is the browsing context from which the auxiliary browsing context was created.

The opener IDL attribute on the Window object, on getting, must return the WindowProxy object of the browsing context from which the current browsing context was created (its opener browsing context), if there is one, if it is still available, and if the current browsing context has not disowned its opener; otherwise, it must return null. On setting, if the new value is null then the current browsing context must disown its opener; if the new value is anything else then the user agent must call the [[DefineOwnProperty]] internal method of the Window object, passing the property name "opener" as the property key, and the Property Descriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true } as the property descriptor, where value is the new value.

7.1.3 Secondary browsing contexts

User agents may support secondary browsing contexts, which are browsing contexts that form part of the user agent's interface, apart from the main content area.

The sidebar link type uses secondary browsing contexts.

7.1.4 Browsing context names

Browsing contexts can have a browsing context name. By default, a browsing context has no name (its name is not set).

A valid browsing context name is any string with at least one character that does not start with a U+005F LOW LINE character. (Names starting with an underscore are reserved for special keywords.)

A valid browsing context name or keyword is any string that is either a valid browsing context 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 summarised in the following (non-normative) table. In this table, "current" means the browsing context that the link or script is in, "parent" means the parent browsing context of the one the link or script is in, "master" means the nearest ancestor browsing context of the one the link or script is in that is not itself in a seamless iframe, "top" means the top-level browsing context of the one the link or script is in, "new" means a new top-level browsing context or auxiliary browsing context is to be created, 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.

Keyword Ordinary effect Effect in an iframe with...
seamless="" sandbox="" sandbox="" seamless="" sandbox="allow-top-navigation" sandbox="allow-top-navigation" seamless=""
none specified, for links and form submissions current master current master current master
empty string current master current master current master
_blank new new maybe new maybe new maybe new maybe new
_self current current current current current current
_parent if there isn't a parent current current current current current current
_parent if parent is also top parent/top parent/top none none parent/top parent/top
_parent if there is one and it's not top parent parent none none none none
_top if top is current current current current current current current
_top if top is not current top top none none top top
name that doesn't exist new new maybe new maybe new maybe new maybe new
name that exists and is a descendant specified descendant specified descendant specified descendant specified descendant specified descendant specified descendant
name that exists and is current current current current current current current
name that exists and is an ancestor that is top specified ancestor specified ancestor none none specified ancestor/top specified ancestor/top
name that exists and is an ancestor that is not top specified ancestor specified ancestor none none none none
other name that exists with common top specified specified none none none none
name that exists with different top, if familiar and one permitted sandboxed navigator specified specified specified specified specified specified
name that exists with different top, if familiar but not one permitted sandboxed navigator specified specified none none none none
name that exists with different top, not familiar new new maybe new maybe new maybe new maybe new

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

7.2 The Window object

[PrimaryGlobal]
/*sealed*/ interface Window : EventTarget {
  // the current browsing context
  [Unforgeable] readonly attribute WindowProxy window;
  [Replaceable] readonly attribute WindowProxy self;
  [Unforgeable] readonly attribute Document document;
           attribute DOMString name; 
  [PutForwards=href, Unforgeable] readonly attribute Location location;
  readonly attribute History history;
  [Replaceable] readonly attribute BarProp locationbar;
  [Replaceable] readonly attribute BarProp menubar;
  [Replaceable] readonly attribute BarProp personalbar;
  [Replaceable] readonly attribute BarProp scrollbars;
  [Replaceable] readonly attribute BarProp statusbar;
  [Replaceable] readonly attribute BarProp toolbar;
           attribute DOMString status;
  void close();
  readonly attribute boolean closed;
  void stop();
  void focus();
  void blur();

  // other browsing contexts
  [Replaceable] readonly attribute WindowProxy frames;
  [Replaceable] readonly attribute unsigned long length;
  [Unforgeable] readonly attribute WindowProxy top;
           attribute any opener;
  readonly attribute WindowProxy parent;
  readonly attribute Element? frameElement;
  WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = "_blank", optional DOMString features = "", optional boolean replace = false);
  getter WindowProxy (unsigned long index);
  getter object (DOMString name);

  // the user agent
  readonly attribute Navigator navigator; 
  [Replaceable] readonly attribute External external;
  readonly attribute ApplicationCache applicationCache;

  // user prompts
  void alert();
  void alert(DOMString message);
  boolean confirm(optional DOMString message = "");
  DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
  void print();
  any showModalDialog(DOMString url, optional any argument);

  void postMessage(any message, DOMString targetOrigin, optional sequence<Transferable> transfer);
};
Window implements GlobalEventHandlers;
Window implements WindowEventHandlers;
window . window
window . frames
window . self

These attributes all return window.

window . document

Returns the Document associated with window.

document . defaultView

Returns the Window object of the active document.

7.2.1 APIs for creating and navigating browsing contexts by name

window = window . open( [ url [, target [, features [, replace ] ] ] ] )

Opens a window to show url (defaults to about:blank), and returns it. The target argument gives the name of the new window. If a window exists with that name already, it is reused. The replace attribute, if true, means that whatever page is currently open in that window will be removed from the window's session history. The features argument is ignored.

window . name [ = value ]

Returns the name of the window.

Can be set, to change the name.

window . close()

Closes the window.

window . closed

Returns true if the window has been closed, false otherwise.

window . stop()

Cancels the document load.

7.2.2 Accessing other browsing contexts

window . length

Returns the number of child browsing contexts.

window[index]

Returns the indicated child browsing context.

7.2.3 Named access on the Window object

window[name]

Returns the indicated element or collection of elements.

As a general rule, relying on this will lead to brittle code. Which IDs end up mapping to this API can vary over time, as new features are added to the Web platform, for example. Instead of this, use document.getElementById() or document.querySelector().

7.2.4 Closing browsing contexts

When the user agent is required to close a browsing context, it must run the following steps:

  1. Let specified browsing context be the browsing context being closed.

  2. Prompt to unload the active document of the specified browsing context. If the user refused to allow the document to be unloaded, then abort these steps.

  3. Unload the active document of the specified browsing context with the recycle parameter set to false.

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

  5. Discard the specified browsing context.

User agents should offer users the ability to arbitrarily close any top-level browsing context.

7.2.5 Browser interface elements

To allow Web pages to integrate with Web browsers, certain Web browser interface elements are exposed in a limited way to scripts in Web pages.

Each interface element is represented by a BarProp object:

interface BarProp {
           attribute boolean visible;
};
window . locationbar . visible

Returns true if the location bar is visible; otherwise, returns false.

window . menubar . visible

Returns true if the menu bar is visible; otherwise, returns false.

window . personalbar . visible

Returns true if the personal bar is visible; otherwise, returns false.

window . scrollbars . visible

Returns true if the scroll bars are visible; otherwise, returns false.

window . statusbar . visible

Returns true if the status bar is visible; otherwise, returns false.

window . toolbar . visible

Returns true if the toolbar is visible; otherwise, returns false.