HTML5

This section describes features that apply most directly to Web browsers. Having said that, except where specified otherwise, the requirements defined in this section do apply to all user agents, whether they are Web browsers or not.

When a browsing context is first created, it must be created with a single Document in its session history, whose address is about:blank, which is marked as being an HTML document, and whose character encoding is UTF-8. The Document must have a single child html node, which itself has a single child body node.

If the browsing context is created specifically to be immediately navigated, then that initial navigation will have replacement enabled.

The origin of the about:blank Document is set when the Document is created. If the new browsing context has a creator browsing context, then the origin of the about:blank Document is the origin of the creator Document. Otherwise, the origin of the about:blank Document is a globally unique identifier assigned when the new browsing context is created.

The top IDL attribute on the Window object of a Document in a browsing context b must return the WindowProxy object of its top-level browsing context (which would be its own WindowProxy object if it was a top-level browsing context itself), if it has one, or its own WindowProxy object otherwise (e.g. if it was a detached nested browsing context).

The parent IDL attribute on the Window object of a Document in a browsing context b must return the WindowProxy object of the parent browsing context, if there is one (i.e. if b is a child browsing context), or the WindowProxy object of the browsing context b itself, otherwise (i.e. if it is a top-level browsing context or a detached nested browsing context).

The frameElement IDL attribute on the Window object of a Document d, on getting, must run the following algorithm:

1. If d is not a Document in a nested browsing context, return null and abort these steps.

2. If the browsing context container's Document does not have the same effective script origin as the entry script, then throw a SECURITY_ERR exception.

3. Otherwise, return the browsing context container for b.

5.1.4 Security

A browsing context A is allowed to navigate a second browsing context B if one of the following conditions is true:

• Either the origin of the active document of A is the same as the origin of the active document of B, or
• The browsing context A is a nested browsing context with a top-level browsing context, and its top-level browsing context is B, or
• The browsing context B is an auxiliary browsing context and A is allowed to navigate B's opener browsing context, or
• The browsing context B is not a top-level browsing context, but there exists an ancestor browsing context of B whose active document has the same origin as the active document of A (possibly in fact being A itself).

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

5.1.5 Groupings of browsing contexts

Each browsing context is defined as having a list of one or more directly reachable browsing contexts. These are:

• The browsing context itself.
• All the browsing context's child browsing contexts.
• The browsing context's parent browsing context.
• All the browsing contexts that have the browsing context as their opener browsing context.
• The browsing context's opener browsing context.

The transitive closure of all the browsing contexts that are directly reachable browsing contexts forms a unit of related browsing contexts.

Each unit of related browsing contexts is then further divided into the smallest number of groups such that every member of each group has an active document with an effective script origin that, through appropriate manipulation of the document.domain attribute, could be made to be the same as other members of the group, but could not be made the same as members of any other group. Each such group is a unit of related similar-origin browsing contexts.

Each unit of related similar-origin browsing contexts can have a entry script which is used to obtain, amongst other things, the script's base URL to resolve relative URLs used in scripts running in that unit of related similar-origin browsing contexts. Initially, there is no entry script. It is changed by the jump to a code entry-point algorithm.

There is at most one event loop per unit of related similar-origin browsing contexts.

The rules for choosing a browsing context given a browsing context name are as follows. The rules assume that they are being applied in the context of a browsing context.

1. If the given browsing context name is the empty string or _self, then the chosen browsing context must be the current one.

   If the given browsing context name is _self, then this is an explicit self-navigation override, which overrides the behavior of the seamless browsing context flag set by the seamless attribute on iframe elements.

2. If the given browsing context name is _parent, then the chosen browsing context must be the parent browsing context of the current one, unless there isn't one, in which case the chosen browsing context must be the current browsing context.

3. If the given browsing context name is _top, then the chosen browsing context must be the top-level browsing context of the current one, if there is one, or else the current browsing context.

4. If the given browsing context name is not _blank and there exists a browsing context whose name is the same as the given browsing context name, and the current browsing context is allowed to navigate that browsing context, and the user agent determines that the two browsing contexts are related enough that it is ok if they reach each other, then that browsing context must be the chosen one. If there are multiple matching browsing contexts, the user agent should select one in some arbitrary consistent manner, such as the most recently opened, most recently focused, or more closely related.

   If the browsing context is chosen by this step to be the current browsing context, then this is also an explicit self-navigation override.

5. Otherwise, a new browsing context is being requested, and what happens depends on the user agent's configuration and/or abilities:

   If the current browsing context had the sandboxed navigation browsing context flag set when its active document was created.

   The user agent may offer to create a new top-level browsing context or reuse an existing top-level browsing context. If the user picks one of those options, then the designated browsing context must be the chosen one (the browsing context's name isn't set to the given browsing context name). Otherwise (if the user agent doesn't offer the option to the user, or if the user declines to allow a browsing context to be used) there must not be a chosen browsing context.

   If the user agent has been configured such that in this instance it will create a new browsing context, and the browsing context is being requested as part of following a hyperlink whose link types include the noreferrer keyword

   A new top-level browsing context must be created. If the given browsing context name is not _blank, then the new top-level browsing context's name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context.

   If it is immediately navigated, then the navigation will be done with replacement enabled.

   If the user agent has been configured such that in this instance it will create a new browsing context, and the noreferrer keyword doesn't apply

   A new auxiliary browsing context must be created, with the opener browsing context being the current one. If the given browsing context name is not _blank, then the new auxiliary browsing context's name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context.

   If it is immediately navigated, then the navigation will be done with replacement enabled.

   If the user agent has been configured such that in this instance it will reuse the current browsing context

   The chosen browsing context is the current browsing context.

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

   There must not be a chosen browsing context.

   User agent implementors are encouraged to provide a way for users to configure the user agent to always reuse the current browsing context.

The window, frames, and self IDL attributes must all return the Window object's browsing context's WindowProxy object.

The document IDL attribute must return the Document object of the Window object's Document's browsing context's active document.

The defaultView IDL attribute of the HTMLDocument interface must return the Document's browsing context's WindowProxy object, if there is one, or null otherwise.

5.2.1 Security

User agents must raise a SECURITY_ERR exception whenever any of the members of a Window object are accessed by scripts whose effective script origin is not the same as the Window object's Document's effective script origin, with the following exceptions:

• The location object
• The postMessage() method
• The frames attribute
• The dynamic nested browsing context properties

When a script whose effective script origin is not the same as the Window object's Document's effective script origin attempts to access that Window object's methods or attributes, the user agent must act as if any changes to the Window object's properties, getters, setters, etc, were not present.

For members that return objects (including function objects), each distinct effective script origin that is not the same as the Window object's Document's effective script origin must be provided with a separate set of objects. These objects must have the prototype chain appropriate for the script for which the objects are created (not those that would be appropriate for scripts whose script's global object is the Window object in question).

The open() method on Window objects provides a mechanism for navigating an existing browsing context or opening and navigating an auxiliary browsing context.

The method has four arguments, though they are all optional.

The first argument, url, must be a valid non-empty URL for a page to load in the browsing context. If no arguments are provided, or if the first argument is the empty string, then the url argument defaults to "about:blank". The argument must be resolved to an absolute URL (or an error), relative to the entry script's base URL, when the method is invoked.

The second argument, target, specifies the name of the browsing context that is to be navigated. It must be a valid browsing context name or keyword. If fewer than two arguments are provided, then the target argument defaults to the value "_blank".

The third argument, features, has no defined effect and is mentioned for historical reasons only. User agents may interpret this argument as instructions to set the size and position of the browsing context, but are encouraged to instead ignore the argument entirely.

The fourth argument, replace, specifies whether or not the new page will replace the page currently loaded in the browsing context, when target identifies an existing browsing context (as opposed to leaving the current page in the browsing context's session history). When three or fewer arguments are provided, replace defaults to false.

When the method is invoked, the user agent must first select a browsing context to navigate by applying the rules for choosing a browsing context given a browsing context name using the target argument as the name and the browsing context of the script as the context in which the algorithm is executed, unless the user has indicated a preference, in which case the browsing context to navigate may instead be the one indicated by the user.

For example, suppose there is a user agent that supports control-clicking a link to open it in a new tab. If a user clicks in that user agent on an element whose onclick handler uses the window.open() API to open a page in an iframe, but, while doing so, holds the control key down, the user agent could override the selection of the target browsing context to instead target a new tab.

Then, the user agent must navigate the selected browsing context to the absolute URL (or error) obtained from resolving url earlier. If the replace is true or if the browsing context was just created as part of the rules for choosing a browsing context given a browsing context name, then replacement must be enabled. The navigation must be done with the browsing context of the entry script as the source browsing context.

The method must return the WindowProxy object of the browsing context that was navigated, or null if no browsing context was navigated.

The name attribute of the Window object must, on getting, return the current name of the browsing context, and, on setting, set the name of the browsing context to the new value.

The name gets reset when the browsing context is navigated to another domain.

The close() method on Window objects should, if the corresponding browsing context A is an auxiliary browsing context that was created by a script (as opposed to by an action of the user), and if the browsing context of the script that invokes the method is allowed to navigate the browsing context A, close the browsing context A (and may discard it too).

The stop() method on Window objects should, if there is an existing attempt to navigate the browsing context and that attempt is not currently running the unload a document algorithm, cancel that navigation and any associated instances of the fetch algorithm. Otherwise, it must do nothing.

The length IDL attribute on the Window interface must return the number of child browsing contexts that are nested through elements that are in the Document that is the active document of that Window object, if that Window's browsing context shares the same event loop as the script's browsing context of the entry script accessing the IDL attribute; otherwise, it must return zero.

The supported property indices on the Window object at any instant are the numbers in the range 0 .. n-1, where n is the number returned by the length IDL attribute. If n is zero then there are no supported property indices.

To determine the value of an indexed property index of a Window object, the user agent must return the WindowProxy object of the indexth child browsing context of the Document that is nested through an element that is in the Document, sorted in the tree order of the elements nesting those browsing contexts.

These properties are the dynamic nested browsing context properties.

The Window interface supports named properties. The supported property names at any moment consist of:

• the value of the name content attribute for all a, applet, area, embed, form, frame, frameset, iframe, img, and object elements in the active document that have a name content attribute, and
• the value of the id content attribute of any HTML element in the active document with an id content attribute.

It is possible that this will change. Browser vendors are considering limiting this behaviour to quirks mode. Read more...

When the Window object is indexed for property retrieval using a name name, then the user agent must return the value obtained using the following steps:

1. Let elements be the list of named elements with the name name in the active document.

   There will be at least one such element, by definition.

2. If elements contains an iframe element, then return the WindowProxy object of the nested browsing context represented by the first such iframe element in tree order, and abort these steps.

3. Otherwise, if elements has only one element, return that element and abort these steps.

4. Otherwise return an HTMLCollection rooted at the Document node, whose filter matches only named elements with the name name.

Named elements with the name name, for the purposes of the above algorithm, are those that are either:

• a, applet, area, embed, form, frame, frameset, iframe, img, or object elements that have a name content attribute whose value is name, or
• HTML elements that have an id content attribute whose value is name.

5.2.5 Garbage collection and browsing contexts

A browsing context has a strong reference to each of its Documents and its WindowProxy object, and the user agent itself has a strong reference to its top-level browsing contexts.

A Document has a strong reference to its Window object.

A Window object has a strong reference to its Document object through its document attribute. Thus, references from other scripts to either of those objects will keep both alive. Similarly, both Document and Window objects have implied strong references to the WindowProxy object.

Each script has a strong reference to its browsing context and its document.

When a browsing context is to discard a Document, the user agent must run the following steps:

1. Set the Document's salvageable state to false.

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

3. Remove any tasks associated with the Document in any task source, without running those tasks.

4. Discard all the child browsing contexts of the Document.

5. Lose the strong reference from the Document's browsing context to the Document.

Whenever a Document object is discarded, it is also removed from the list of the worker's Documents of each worker whose list contains that Document.

When a browsing context is discarded, the strong reference from the user agent itself to the browsing context must be severed, and all the Document objects for all the entries in the browsing context's session history must be discarded as well.

User agents may discard top-level browsing contexts at any time (typically, in response to user requests, e.g. when a user closes a window containing one or more top-level browsing contexts). Other browsing contexts must be discarded once their WindowProxy object is eligible for garbage collection.

The visible attribute, on getting, must return either true or a value determined by the user agent to most accurately represent the visibility state of the user interface element that the object represents, as described below. On setting, the new value must be discarded.

The following BarProp objects exist for each Document object in a browsing context. Some of the user interface elements represented by these objects might have no equivalent in some user agents; for those user agents, except when otherwise specified, the object must act as if it was present and visible (i.e. its visible attribute must return true).

The location bar BarProp object

Represents the user interface element that contains a control that displays the URL of the active document, or some similar interface concept.

The menu bar BarProp object

Represents the user interface element that contains a list of commands in menu form, or some similar interface concept.

The personal bar BarProp object

Represents the user interface element that contains links to the user's favorite pages, or some similar interface concept.

The scrollbar BarProp object

Represents the user interface element that contains a scrolling mechanism, or some similar interface concept.

The status bar BarProp object

Represents a user interface element found immediately below or after the document, as appropriate for the user's media. If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e. its visible attribute may return false).

The toolbar BarProp object

Represents the user interface element found immediately above or before the document, as appropriate for the user's media. If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e. its visible attribute may return false).

The locationbar attribute must return the location bar BarProp object.

The menubar attribute must return the menu bar BarProp object.

The personalbar attribute must return the personal bar BarProp object.

The scrollbars attribute must return the scrollbar BarProp object.

The statusbar attribute must return the status bar BarProp object.

The toolbar attribute must return the toolbar BarProp object.

5.2.7 The WindowProxy object

As mentioned earlier, each browsing context has a WindowProxy object. This object is unusual in that all operations that would be performed on it must be performed on the Window object of the browsing context's active document instead. It is thus indistinguishable from that Window object in every way until the browsing context is navigated.

There is no WindowProxy interface object.

The WindowProxy object allows scripts to act as if each browsing context had a single Window object, while still keeping separate Window objects for each Document.

var x = window;
x instanceof Window; // true
x === this; // true