HTML5

6.1.2 Enabling and disabling scripting

Scripting is enabled in a browsing context when all of the following conditions are true:

• The user agent supports scripting.
• The user has not disabled scripting for this browsing context at this time. (User agents may provide users with the option to disable scripting globally, or in a finer-grained manner, e.g. on a per-origin basis.)
• The browsing context's active document's active sandboxing flag set does not have its sandboxed scripts browsing context flag set.

Scripting is disabled in a browsing context when any of the above conditions are false (i.e. when scripting is not enabled).

Scripting is enabled for a node if the Document object of the node (the node itself, if it is itself a Document object) has an associated browsing context, and scripting is enabled in that browsing context.

Scripting is disabled for a node if there is no such browsing context, or if scripting is disabled in that browsing context.

6.1.3 Processing model

6.1.3.1 Definitions

This specification describes three kinds of JavaScript global environments: the document environment, the dedicated worker environment, and the shared worker environment. The dedicated worker environment and the shared worker environment are both types of worker environments.

Except where otherwise specified, a JavaScript global environment is a document environment.

A script has:

A code entry-point

A code entry-point represents a block of executable code that the script exposes to other scripts and to the user agent. Typically, the code corresponding to the code entry-point is executed immediately after the script is parsed, but for event handlers, it is called each time the handler is invoked.

In JavaScript script blocks, this corresponds to the execution context of the global code.

Optionally, a muted errors flag

A flag which, if set, means that error information will not be provided for errors in this script (used to mute errors for cross-origin scripts, since that can leak private information).

A settings object

A script settings object, various settings that are shared with other scripts in the same context.

A script settings object specifies algorithms for obtaining the following:

A script execution environment for each language supported by the user agent

The characteristics of the script execution environment depend on the language, and are not defined by this specification.

In JavaScript, the script execution environment consists of the interpreter, the stack of execution contexts, the global code and function code and the Function objects resulting, and so forth.

A global object

An object that provides the APIs that can be called by the code in scripts that use this settings object.

This is typically a Window object or a WorkerGlobalScope object. When a global object is an empty object, it can't do anything that interacts with the environment.

If the global object is a Window object, then, in JavaScript, the ThisBinding of the global execution context for this script must be the Window object's WindowProxy object, rather than the global object. [ECMA262]

This is a willful violation of the JavaScript specification current at the time of writing (ECMAScript edition 5, as defined in section 10.4.1.1 Initial Global Execution Context, step 3). The JavaScript specification requires that the this keyword in the global scope return the global object, but this is not compatible with the security design prevalent in implementations as specified herein. [ECMA262]

A responsible browsing context

A browsing context that is assigned responsibility for actions taken by the scripts that use this script settings object.

When a script creates and navigates a new top-level browsing context, the opener attribute of the new browsing context's Window object will be set to the responsible browsing context's WindowProxy object.

A responsible document

A Document that is assigned responsibility for actions taken by the scripts that use this script settings object.

For example, the address of the responsible document is used to set the address of any Documents created using createDocument().

A responsible event loop

An event loop that is used when it would not be immediately clear what event loop to use.

An API referrer source

Either a Document (specifically, the responsible document), or a URL, which is used by some APIs to determine what value to use for the Referer (sic) header in calls to the fetching algorithm.

An API URL character encoding

A character encoding used to encode URLs by APIs called by scripts that use this script settings object.

An API base URL

An absolute URL used by APIs called by scripts that use this script settings object to resolve relative URLs.

An origin and an effective script origin

An instrument used in security checks.

The relevant settings object for a global object o is the script settings object whose global object is o. (There is always a 1:1 mapping of global objects to script settings objects.)

The relevant settings object for a script s is the settings object of s.

6.1.3.2 Script settings for browsing contexts

Whenever a new Window object is created, it must also create a script settings object whose algorithms are defined as follows:

The script execution environments

When the script settings object is created, for each language supported by the user agent, create an appropriate execution environment as defined by the relevant specification.

When a script execution environment is needed, return the appropriate one from those created when the script settings object was created.

The global object

Return the Window object itself.

The responsible browsing context

Return the browsing context with which the Window object is associated.

The responsible document

Return the Document with which the Window is currently associated.

The responsible event loop

Return the event loop that is associated with the unit of related similar-origin browsing contexts to which the Window object's browsing context belongs.

The API referrer source

Return the Document with which the Window is currently associated.

The API URL character encoding

Return the current character encoding of the Document with which the Window is currently associated.

The API base URL

Return the current base URL of the Document with which the Window is currently associated.

The origin

Return the origin of the Document with which the Window is currently associated.

The effective script origin

Return the effective script origin of the Document with which the Window is currently associated.

6.1.3.3 Calling scripts

Each unit of related similar-origin browsing contexts has a stack of script settings objects, which must be initially empty. When a new script settings object is pushed onto this stack, the specified script settings object is to be added to the stack; when the script settings object on this stack that was most recently pushed onto it is to be popped from the stack, it must be removed. Entries on this stack can be labeled as candidate entry settings objects.

When a user agent is to jump to a code entry-point for a script s, the user agent must run the following steps:

1. Let context be the settings object of s.

2. Prepare to run a callback with context as the script settings object. If this returns "do not run" then abort these steps.

3. Make the appropriate script execution environment specified by context execute the s's code entry-point.

4. Clean up after running a callback.

The steps to prepare to run a callback with a script settings object o are as follows. They return either "run" or "do not run".

1. If the global object specified by o is a Window object whose Document object is not fully active, then return "do not run" and abort these steps.

2. If scripting is disabled for the responsible browsing context specified by o, then return "do not run" and abort these steps.

3. Push o onto the stack of script settings objects, and label it as a candidate entry settings object.

4. Return "run".

The steps to clean up after running a callback are as follows:

1. Pop the current incumbent settings object from the stack of script settings objects.

2. If the stack of script settings objects is now empty, run the global script clean-up jobs. (These cannot run scripts.)

3. If the stack of script settings objects is now empty, perform a microtask checkpoint. (If this runs scripts, these algorithms will be invoked reentrantly.)

These algorithms are not invoked by one script directly calling another, but they can be invoked reentrantly in an indirect manner, e.g. if a script dispatches an event which has event listeners registered.

When a JavaScript SourceElements production is to be evaluated, the settings object of the script corresponding to that SourceElements must be pushed onto the stack of script settings objects before the evaluation begins, and popped when the evaluation ends (regardless of whether it's an abrupt completion or not).

The entry settings object is the most-recently added script settings object in the stack of script settings objects that is labeled as a candidate entry settings object. If the stack is empty, or has no entries labeled as such, then there is no entry settings object. It is used to obtain, amongst other things, the API base URL to resolve relative URLs used in scripts running in that unit of related similar-origin browsing contexts.

The incumbent settings object is the script settings object in the stack of script settings objects that was most-recently added (i.e. the last one on the stack). If the stack is empty, then there is no incumbent settings object. It is used in some security checks.

The WebIDL specification also uses these algorithms. [WEBIDL]

Each unit of related similar-origin browsing contexts has a performing a microtask checkpoint flag, which must initially be false. It is used to prevent reentrant invocation of the algorithm to invoke MutationObserver objects. For the purposes of MutationObserver objects, each unit of related similar-origin browsing contexts is a distinct scripting environment.

Each unit of related similar-origin browsing contexts has a global script clean-up jobs list, which must initially be empty. A global script clean-up job cannot run scripts, and cannot be sensitive to the order in which other clean-up jobs are executed. The File API uses this to release blob: URLs. [FILEAPI]

When the user agent is to run the global script clean-up jobs, the user agent must perform each of the jobs in the global script clean-up jobs list and then empty the list.

6.1.3.4 Creating scripts

When the specification says that a script is to be created, given some script source, a script source URL, its scripting language, a script settings object, and optionally a muted errors flag, the user agent must run the following steps:

1. Let script be a new script that this algorithm will subsequently initialize.

2. If scripting is disabled for browsing context passed to this algorithm, then abort these steps, as if the script source described a program that did nothing but return void.

3. Obtain the appropriate script execution environment for the given scripting language from the script settings object provided.

4. Parse/compile/initialize the source of the script using the script execution environment, as appropriate for the scripting language, and thus obtain script's code entry-point.

5. Let script's settings object be the script settings object provided.

6. If the muted errors flag was set, then set script's muted errors flag.

7. If all the steps above succeeded (in particular, if the script was compiled successfully), Jump to script's code entry-point.

   Otherwise, report the error for script, with the problematic position (line number and column number), using the global object specified by the script settings object as the target. If the error is still not handled after this, then the error may be reported to the user.

6.1.3.5 Killing scripts

User agents may impose resource limitations on scripts, for example CPU quotas, memory limits, total execution time limits, or bandwidth limitations. When a script exceeds a limit, the user agent may either throw a QuotaExceededError exception, abort the script without an exception, prompt the user, or throttle script execution.

<script>
 while (true) { /* loop */ }
</script>

User agents are encouraged to allow users to disable scripting whenever the user is prompted either by a script (e.g. using the window.alert() API) or because of a script's actions (e.g. because it has exceeded a time limit).

If scripting is disabled while a script is executing, the script should be terminated immediately.

User agents may allow users to specifically disable scripts just for the purposes of closing a browsing context.

For example, the prompt mentioned in the example above could also offer the user with a mechanism to just close the page entirely, without running any unload event handlers.

6.1.3.6 Runtime script errors

When the user agent is required to report an error for a particular script script with a particular position line:col, using a particular target target, it must run these steps, after which the error is either handled or not handled:

1. If target is in error reporting mode, then abort these steps; the error is not handled.

2. Let target be in error reporting mode.

3. Let message be a user-agent-defined string describing the error in a helpful manner.

4. Let error object be the object that represents the error: in the case of an uncaught exception, that would be the object that was thrown; in the case of a JavaScript error that would be an Error object. If there is no corresponding object, then the null value must be used instead.

5. Let location be an absolute URL that corresponds to the resource from which script was obtained.

   The resource containing the script will typically be the file from which the Document was parsed, e.g. for inline script elements or event handler content attributes; or the JavaScript file that the script was in, for external scripts. Even for dynamically-generated scripts, user agents are strongly encouraged to attempt to keep track of the original source of a script. For example, if an external script uses the document.write() API to insert an inline script element during parsing, the URL of the resource containing the script would ideally be reported as being the external script, and the line number might ideally be reported as the line with the document.write() call or where the string passed to that call was first constructed. Naturally, implementing this can be somewhat non-trivial.

   User agents are similarly encouraged to keep careful track of the original line numbers, even in the face of document.write() calls mutating the document as it is parsed, or event handler content attributes spanning multiple lines.

6. If script has muted errors, then set message to "Script error.", set location to the empty string, set line and col to 0, and set error object to null.

7. Let event be a new trusted ErrorEvent object that does not bubble but is cancelable, and which has the event name error.

8. Initialize event's message attribute to message.

9. Initialize event's filename attribute to location.

10. Initialize event's lineno attribute to line.

11. Initialize event's colno attribute to col.

12. Initialize event's error attribute to error object.

13. Dispatch event at target.

14. Let target no longer be in error reporting mode.

15. If event was canceled, then the error is handled. Otherwise, the error is not handled.

6.1.3.6.1 Runtime script errors in documents

Whenever an uncaught runtime script error occurs in one of the scripts associated with a Document, the user agent must report the error for the relevant script, with the problematic position (line number and column number) in the resource containing the script, using the global object specified by the script's settings object as the target. If the error is still not handled after this, then the error may be reported to the user.

6.1.4 Event loops

6.1.4.1 Definitions

To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section.

There must be at least one event loop per user agent, and at most one event loop per unit of related similar-origin browsing contexts.

When there is more than one event loop for a unit of related browsing contexts, complications arise when a browsing context in that group is navigated such that it switches from one unit of related similar-origin browsing contexts to another. This specification does not currently describe how to handle these complications.

An event loop always has at least one browsing context. If an event loop's browsing contexts all go away, then the event loop goes away as well. A browsing context always has an event loop coordinating its activities.

An event loop has one or more task queues. A task queue is an ordered list of tasks, which are algorithms that are responsible for such work as:

Events

Asynchronously dispatching an Event object at a particular EventTarget object is often done by a dedicated task.

Not all events are dispatched using the task queue, many are dispatched synchronously during other tasks.

Parsing

The HTML parser tokenizing one or more bytes, and then processing any resulting tokens, is typically a task.

Callbacks

Calling a callback asynchronously is often done by a dedicated task.

Using a resource

When an algorithm fetches a resource, if the fetching occurs asynchronously then the processing of the resource once some or all of the resource is available is performed by a task.

Reacting to DOM manipulation

Some elements have tasks that trigger in response to DOM manipulation, e.g. when that element is inserted into the document.

Each task is associated with a Document; if the task was queued in the context of an element, then it is the element's Document; if the task was queued in the context of a browsing context, then it is the browsing context's active document at the time the task was queued; if the task was queued by or for a script then the document is the responsible document specified by the script's settings object.

A task is intended for a specific event loop: the event loop that is handling tasks for the task's associated Document.

When a user agent is to queue a task, it must add the given task to one of the task queues of the relevant event loop.

Each task is defined as coming from a specific task source. All the tasks from one particular task source and destined to a particular event loop (e.g. the callbacks generated by timers of a Document, the events fired for mouse movements over that Document, the tasks queued for the parser of that Document) must always be added to the same task queue, but tasks from different task sources may be placed in different task queues.

For example, a user agent could have one task queue for mouse and key events (the user interaction task source), and another for everything else. The user agent could then give keyboard and mouse events preference over other tasks three quarters of the time, keeping the interface responsive but not starving other task queues, and never processing events from any one task source out of order.

A user agent may have one storage mutex. This mutex is used to control access to shared state like cookies. At any one point, the storage mutex is either free, or owned by a particular event loop or instance of the fetching algorithm.

If a user agent does not implement a storage mutex, it is exempt from implementing the requirements that require it to acquire or release it.

User agent implementors have to make a choice between two evils. On the one hand, not implementing the storage mutex means that there is a risk of data corruption: a site could, for instance, try to read a cookie, increment its value, then write it back out, using the new value of the cookie as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects. On the other hand, implementing the storage mutex has potentially serious performance implications: whenever a site uses Web Storage or cookies, all other sites that try to use Web Storage or cookies are blocked until the first site finishes.

Whenever a script calls into a plugin, and whenever a plugin calls into a script, the user agent must release the storage mutex.

6.1.4.2 Processing model

An event loop must continually run through the following steps for as long as it exists:

1. Run the oldest task on one of the event loop's task queues, if any, ignoring tasks whose associated Documents are not fully active. The user agent may pick any task queue.

2. If the storage mutex is now owned by the event loop, release it so that it is once again free.

3. If a task was run in the first step above, remove that task from its task queue.

4. If this event loop is not a worker's event loop, run these substeps:

   1. Perform a microtask checkpoint.

   2. Provide a stable state.

   3. If necessary, update the rendering or user interface of any Document or browsing context to reflect the current state.

5. Otherwise, if this event loop is running for a WorkerGlobalScope, but there are no events in the event loop's task queues and the WorkerGlobalScope object's closing flag is true, then destroy the event loop, aborting these steps.

6. Return to the first step of the event loop.

When a user agent is to perform a microtask checkpoint, if the performing a microtask checkpoint flag is false, then the user agent must run the following steps:

1. Let the performing a microtask checkpoint flag be true.

2. Perform a custom elements checkpoint. [CUSTOM]

3. Sort the tables with pending sorts.

4. Invoke MutationObserver objects for the unit of related similar-origin browsing contexts to which the responsible browsing context specified by the script's settings object belongs, using the task wrapper algorithm as the steps to invoke each callback.

   This will typically invoke scripted callbacks, which eventually calls the clean up after running a callback steps, which call this perform a microtask checkpoint algorithm again, which is why we use the performing a microtask checkpoint flag to avoid reentrancy.

5. Let the performing a microtask checkpoint flag be false.

When the user agent is to provide a stable state, if any asynchronously-running algorithms are awaiting a stable state, then the user agent must run their synchronous section and then resume running their asynchronous algorithm (if appropriate).

A synchronous section never mutates the DOM, runs any script, or has any side-effects detectable from another synchronous section, and thus synchronous sections can be run in any order, and cannot spin the event loop.

Steps in synchronous sections are marked with ⌛.

The task wrapper algorithm, which is implicitly invoked in the context of an event loop and is used to invoke a given callback in a specific way, is as follows:

1. Invoke callback as specified.

The above will change in due course; see bug 20821.

When an algorithm says to spin the event loop until a condition goal is met, the user agent must run the following steps:

1. Let task source be the task source of the currently running task.

2. Let old stack of script settings objects be a copy of the stack of script settings objects.

3. Empty the stack of script settings objects.

4. Run the global script clean-up jobs.

5. Perform a microtask checkpoint.

6. Stop the currently running task, allowing the event loop to resume, but continue these steps asynchronously.

   This causes the event loop to move on to the second step of its processing model (defined above).

7. Wait until the condition goal is met.

8. Queue a task to continue running these steps, using the task source task source. Wait until this task runs before continuing these steps.

9. Replace the stack of script settings objects with the old stack of script settings objects.

10. Return to the caller.

Some of the algorithms in this specification, for historical reasons, require the user agent to pause while running a task until a condition goal is met. This means running the following steps:

1. If any asynchronously-running algorithms are awaiting a stable state, then run their synchronous section and then resume running their asynchronous algorithm. (See the event loop processing model definition above for details.)

2. If necessary, update the rendering or user interface of any Document or browsing context to reflect the current state.

3. Wait until the condition goal is met. While a user agent has a paused task, the corresponding event loop must not run further tasks, and any script in the currently running task must block. User agents should remain responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be doing anything.

When a user agent is to obtain the storage mutex as part of running a task, it must run through the following steps:

1. If the storage mutex is already owned by this task's event loop, then abort these steps.

2. Otherwise, pause until the storage mutex can be taken by the event loop.

3. Take ownership of the storage mutex.

6.1.4.3 Generic task sources

The following task sources are used by a number of mostly unrelated features in this and other specifications.

The DOM manipulation task source

This task source is used for features that react to DOM manipulations, such as things that happen asynchronously when an element is inserted into the document.

The user interaction task source

This task source is used for features that react to user interaction, for example keyboard or mouse input.

Asynchronous events sent in response to user input (e.g. click events) must be fired using tasks queued with the user interaction task source. [DOMEVENTS]

The networking task source

This task source is used for features that trigger in response to network activity.

The history traversal task source

This task source is used to queue calls to history.back() and similar APIs.

An event handler IDL attribute is an IDL attribute for a specific event handler. The name of the IDL attribute is the same as the name of the event handler.

Event handler IDL attributes, on setting, must set the corresponding event handler to their new value, and on getting, must return the result of getting the current value of the event handler in question (this can throw an exception, in which case the getting propagates it to the caller, it does not catch it).

If an event handler IDL attribute exposes an event handler of an object that doesn't exist, it must always return null on getting and must do nothing on setting.

This can happen in particular for event handler IDL attribute on body elements that do not have corresponding Window objects.

Certain event handler IDL attributes have additional requirements, in particular the onmessage attribute of MessagePort objects.

When an event handler content attribute is set, the user agent must set the corresponding event handler to an internal raw uncompiled handler consisting of the attribute's new value and the script location where the attribute was set to this value

When an event handler content attribute is removed, the user agent must set the corresponding event handler to null.

When an event handler H of an element or object T implementing the EventTarget interface is first set to a non-null value, the user agent must append an event listener to the list of event listeners associated with T with type set to the event handler event type corresponding to H, capture set to false, and listener set to the event handler processing algorithm defined below. [DOM]

The listener is emphatically not the event handler itself. Every event handler ends up registering the same listener, the algorithm defined below, which takes care of invoking the right callback, and processing the callback's return value.

This only happens the first time the event handler's value is set. Since listeners are called in the order they were registered, the order of event listeners for a particular event type will always be first the event listeners registered with addEventListener() before the first time the event handler was set to a non-null value, then the callback to which it is currently set, if any, and finally the event listeners registered with addEventListener() after the first time the event handler was set to a non-null value.

The interfaces implemented by the event object do not influence whether an event handler is triggered or not.

The event handler processing algorithm for an event handler H and an Event object E is as follows:

1. Let callback be the result of getting the current value of the event handler H.

2. If callback is null, then abort these steps.

3. Process the Event object E as follows:

   If E is an ErrorEvent object and the event handler IDL attribute's type is OnErrorEventHandler

   Invoke callback with five arguments, the first one having the value of E's message attribute, the second having the value of E's filename attribute, the third having the value of E's lineno attribute, the fourth having the value of E's colno attribute, the fifth having the value of E's error attribute, and with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

   Otherwise

   Invoke callback with one argument, the value of which is the Event object E, with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

   In this step, invoke means to run the jump to a code entry-point algorithm.

4. Process return value as follows:

   If the event type is mouseover

   If the event type is error and E is an ErrorEvent object

   If return value is a WebIDL boolean true value, then cancel the event.

   If the event type is beforeunload

   The event handler IDL attribute's type is OnBeforeUnloadEventHandler, and the return value will therefore have been coerced into either the value null or a DOMString.

   If the return value is null, then cancel the event.

   Otherwise, If the Event object E is a BeforeUnloadEvent object, and the Event object E's returnValue attribute's value is the empty string, then set the returnValue attribute's value to return value.

   Otherwise

   If return value is a WebIDL boolean false value, then cancel the event.

An internal raw uncompiled handler is a tuple with the following information:

• An uncompiled script body
• A location where the script body originated, in case an error needs to be reported

When the user agent is to get the current value of the event handler H, it must run these steps:

1. If H's value is an internal raw uncompiled handler, run these substeps:

   1. If H is an element's event handler, then let element be the element, and document be the element's Document.

      Otherwise, H is a Window object's event handler: let element be null, and let document be the Document most recently associated with that Window object.

   2. If document is not in a browsing context, or if scripting is enabled for document's browsing context, then return null and abort the algorithm for getting the current value of the event handler.

   3. Let body be the uncompiled script body in the internal raw uncompiled handler.

   4. Let location be the location where the script body originated, as given by the internal raw uncompiled handler.

   5. If element is not null and element has a form owner, let form owner be that form owner. Otherwise, let form owner be null.

   6. Let script settings be the script settings object created for the Window object with which document is currently associated.

   7. Obtain the script execution environment for JavaScript from script settings.

   8. If body is not parsable as FunctionBody or if parsing detects an early error, then follow these substeps:

      1. Set H's value to null.

      2. Report the error for the appropriate script and with the appropriate position (line number and column number) given by location, using the global object specified by script settings as the target. If the error is still not handled after this, then the error may be reported to the user.

      3. Jump to the step labeled end below.

      FunctionBody is defined in ECMAScript edition 5 section 13 Function Definition. Early error is defined in ECMAScript edition 5 section 16 Errors. [ECMA262]

   9. If body begins with a Directive Prologue that contains a Use Strict Directive then let strict be true, otherwise let strict be false.

      The terms "Directive Prologue" and "Use Strict Directive" are defined in ECMAScript edition 5 section 14.1 Directive Prologues and the Use Strict Directive. [ECMA262]

   10. Using the script execution environment obtained above, create a function object (as defined in ECMAScript edition 5 section 13.2 Creating Function Objects), with:

       Parameter list FormalParameterList

       If H is an onerror event handler of a Window object

       Let the function have five arguments, named event, source, lineno, colno, and error.

       Otherwise

       Let the function have a single argument called event.

       Function body FunctionBody

       The result of parsing body above.

       Lexical Environment Scope

       1. Let Scope be the result of NewObjectEnvironment(document, the global environment).
       2. If form owner is not null, let Scope be the result of NewObjectEnvironment(>form owner, Scope).
       3. If element is not null, let Scope be the result of NewObjectEnvironment(element, Scope).

       NewObjectEnvironment() is defined in ECMAScript edition 5 section 10.2.2.3 NewObjectEnvironment (O, E). [ECMA262]

       Boolean flag Strict

       The value of strict.

       Let function be this new function.

   11. Let script be a new script.

   12. Let script's code entry-point be function.

   13. Let script's settings object be script settings.

   14. Set H to function.

2. End: Return H's value.

6.1.5.3 Event firing

Certain operations and methods are defined as firing events on elements. For example, the click() method on the HTMLElement interface is defined as firing a click event on the element. [DOMEVENTS]

Firing a simple event named e means that a trusted event with the name e, which does not bubble (except where otherwise stated) and is not cancelable (except where otherwise stated), and which uses the Event interface, must be created and dispatched at the given target.

Firing a synthetic mouse event named e means that an event with the name e, which is trusted (except where otherwise stated), does not bubble (except where otherwise stated), is not cancelable (except where otherwise stated), and which uses the MouseEvent interface, must be created and dispatched at the given target. The event object must have its screenX, screenY, clientX, clientY, and button attributes initialized to 0, its ctrlKey, shiftKey, altKey, and metaKey attributes initialized according to the current state of the key input device, if any (false for any keys that are not available), its detail attribute initialized to 1, and its relatedTarget attribute initialized to null (except where otherwise stated). The getModifierState() method on the object must return values appropriately describing the state of the key input device at the time the event is created.

Firing a click event means firing a synthetic mouse event named click, which bubbles and is cancelable.

The default action of these events is to do nothing except where otherwise stated.

6.1.5.4 Events and the Window object

When an event is dispatched at a DOM node in a Document in a browsing context, if the event is not a load event, the user agent must act as if, for the purposes of event dispatching, the Window object is the parent of the Document object. [DOM]

The WindowBase64 interface adds to the Window interface and the WorkerGlobalScope interface (part of Web workers).

The btoa() method must throw an InvalidCharacterError exception if the method's first argument contains any character whose code point is greater than U+00FF. Otherwise, the user agent must convert that argument to a sequence of octets whose nth octet is the eight-bit representation of the code point of the nth character of the argument, and then must apply the base64 algorithm to that sequence of octets, and return the result. [RFC4648]

The atob() method must run the following steps to parse the string passed in the method's first argument:

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Remove all space characters from input.

4. If the length of input divides by 4 leaving no remainder, then: if input ends with one or two "=" (U+003D) characters, remove them from input.

5. If the length of input divides by 4 leaving a remainder of 1, throw an InvalidCharacterError exception and abort these steps.

6. If input contains a character that is not in the following list of characters and character ranges, throw an InvalidCharacterError exception and abort these steps:

   • "+" (U+002B)
   • "/" (U+002F)
   • Alphanumeric ASCII characters

7. Let output be a string, initially empty.

8. Let buffer be a buffer that can have bits appended to it, initially empty.

9. While position does not point past the end of input, run these substeps:

   1. Find the character pointed to by position in the first column of the following table. Let n be the number given in the second cell of the same row.

      Character	Number
      A	0
      B	1
      C	2
      D	3
      E	4
      F	5
      G	6
      H	7
      I	8
      J	9
      K	10
      L	11
      M	12
      N	13
      O	14
      P	15
      Q	16
      R	17
      S	18
      T	19
      U	20
      V	21
      W	22
      X	23
      Y	24
      Z	25
      a	26
      b	27
      c	28
      d	29
      e	30
      f	31
      g	32
      h	33
      i	34
      j	35
      k	36
      l	37
      m	38
      n	39
      o	40
      p	41
      q	42
      r	43
      s	44
      t	45
      u	46
      v	47
      w	48
      x	49
      y	50
      z	51
      0	52
      1	53
      2	54
      3	55
      4	56
      5	57
      6	58
      7	59
      8	60
      9	61
      +	62
      /	63

   2. Append to buffer the six bits corresponding to number, most significant bit first.

   3. If buffer has accumulated 24 bits, interpret them as three 8-bit big-endian numbers. Append the three characters with code points equal to those numbers to output, in the same order, and then empty buffer.

   4. Advance position by one character.

10. If buffer is not empty, it contains either 12 or 18 bits. If it contains 12 bits, discard the last four and interpret the remaining eight as an 8-bit big-endian number. If it contains 18 bits, discard the last two and interpret the remaining 16 as two 8-bit big-endian numbers. Append the one or two characters with code points equal to those one or two numbers to output, in the same order.

    The discarded bits mean that, for instance, atob("YQ") and atob("YR") both return "a".

11. Return output.

Document objects have an ignore-opens-during-unload counter, which is used to prevent scripts from invoking the document.open() method (directly or indirectly) while the document is being unloaded. Initially, the counter must be set to zero.

When called with two arguments, the document.open() method must act as follows:

1. If the Document object is not flagged as an HTML document, throw an InvalidStateError exception and abort these steps.

2. If the Document object is not an active document, then abort these steps.

3. Let type be the value of the first argument.

4. If the second argument is an ASCII case-insensitive match for the value "replace", then let replace be true.

   Otherwise, if the browsing context's session history contains only one Document, and that was the about:blank Document created when the browsing context was created, and that Document has never had the unload a document algorithm invoked on it (e.g. by a previous call to document.open()), then let replace be true.

   Otherwise, let replace be false.

5. If the Document has an active parser whose script nesting level is greater than zero, then the method does nothing. Abort these steps and return the Document object on which the method was invoked.

   This basically causes document.open() to be ignored when it's called in an inline script found during parsing, while still letting it have an effect when called asynchronously.

6. Similarly, if the Document's ignore-opens-during-unload counter is greater than zero, then the method does nothing. Abort these steps and return the Document object on which the method was invoked.

   This basically causes document.open() to be ignored when it's called from a beforeunload pagehide, or unload event handler while the Document is being unloaded.

7. Release the storage mutex.

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

9. Prompt to unload the Document object. If the user refused to allow the document to be unloaded, then abort these steps and return the Document object on which the method was invoked.

10. Unload the Document object, with the recycle parameter set to true.

11. Abort the Document.

12. Unregister all event listeners registered on the Document node and its descendants.

13. Remove any tasks associated with the Document in any task source.

14. Remove all child nodes of the document, without firing any mutation events.

15. Replace the Document's singleton objects with new instances of those objects. (This includes in particular the Window, Location, History, ApplicationCache, and Navigator, objects, the various BarProp objects, the two Storage objects, the various HTMLCollection objects, and objects defined by other specifications, like Selection and the document's UndoManager. It also includes all the Web IDL prototypes in the JavaScript binding, including the Document object's prototype.)

    The new Window object has a new script settings object.

16. Change the document's character encoding to UTF-8.

17. If the Document is ready for post-load tasks, then set the Document object's reload override flag and set the Document's reload override buffer to the empty string.

18. Set the Document's salvageable state back to true.

19. Change the document's address to the address of the responsible document specified by the entry settings object.

20. If the Document's iframe load in progress flag is set, set the Document's mute iframe load flag.

21. Create a new HTML parser and associate it with the document. This is a script-created parser (meaning that it can be closed by the document.open() and document.close() methods, and that the tokenizer will wait for an explicit call to document.close() before emitting an end-of-file token). The encoding confidence is irrelevant.

22. Set the current document readiness of the document to "loading".

23. If type is an ASCII case-insensitive match for the string "replace", then, for historical reasons, set it to the string "text/html".

    Otherwise:

    If the type string contains a ";" (U+003B) character, remove the first such character and all characters from it up to the end of the string.

    Strip leading and trailing whitespace from type.

24. If type is not now an ASCII case-insensitive match for the string "text/html", then act as if the tokenizer had emitted a start tag token with the tag name "pre" followed by a single "LF" (U+000A) character, then switch the HTML parser's tokenizer to the PLAINTEXT state.

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

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

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

27. Remove any earlier entries that share the same Document.

28. If replace is false, then add a new entry, just before the last entry, and associate with the new entry the text that was parsed by the previous parser associated with the Document object, as well as the state of the document at the start of these steps. This allows the user to step backwards in the session history to see the page before it was blown away by the document.open() call. This new entry does not have a Document object, so a new one will be created if the session history is traversed to that entry.

29. Finally, set the insertion point to point at just before the end of the input stream (which at this point will be empty).

30. Return the Document on which the method was invoked.

The document.open() method does not affect whether a Document is ready for post-load tasks or completely loaded.

When called with four arguments, the open() method on the Document object must call the open() method on the Window object of the Document object, with the same arguments as the original call to the open() method, and return whatever that method returned. If the Document object has no Window object, then the method must throw an InvalidAccessError exception.

The close() method must run the following steps:

1. If the Document object is not flagged as an HTML document, throw an InvalidStateError exception and abort these steps.

2. If there is no script-created parser associated with the document, then abort these steps.

3. Insert an explicit "EOF" character at the end of the parser's input stream.

4. If there is a pending parsing-blocking script, then abort these steps.

5. Run the tokenizer, processing resulting tokens as they are emitted, and stopping when the tokenizer reaches the explicit "EOF" character or spins the event loop.

Document objects have an ignore-destructive-writes counter, which is used in conjunction with the processing of script elements to prevent external scripts from being able to use document.write() to blow away the document by implicitly calling document.open(). Initially, the counter must be set to zero.

The document.write(...) method must act as follows:

1. If the method was invoked on an XML document, throw an InvalidStateError exception and abort these steps.

2. If the Document object is not an active document, then abort these steps.

3. If the insertion point is undefined and either the Document's ignore-opens-during-unload counter is greater than zero or the Document's ignore-destructive-writes counter is greater than zero, abort these steps.

4. If the insertion point is undefined, call the open() method on the document object (with no arguments). If the user refused to allow the document to be unloaded, then abort these steps. Otherwise, the insertion point will point at just before the end of the (empty) input stream.

5. Insert the string consisting of the concatenation of all the arguments to the method into the input stream just before the insertion point.

6. If the Document object's reload override flag is set, then append the string consisting of the concatenation of all the arguments to the method to the Document's reload override buffer.

7. If there is no pending parsing-blocking script, have the HTML parser process the characters that were inserted, one at a time, processing resulting tokens as they are emitted, and stopping when the tokenizer reaches the insertion point or when the processing of the tokenizer is aborted by the tree construction stage (this can happen if a script end tag token is emitted by the tokenizer).

   If the document.write() method was called from script executing inline (i.e. executing because the parser parsed a set of script tags), then this is a reentrant invocation of the parser.

8. Finally, return from the method.

The document.writeln(...) method, when invoked, must act as if the document.write() method had been invoked with the same argument(s), plus an extra argument consisting of a string containing a single line feed character (U+000A).

The WindowTimers interface adds to the Window interface and the WorkerGlobalScope interface (part of Web workers).

Each object that implements the WindowTimers interface has a list of active timers. Each entry in this lists is identified by a number, which must be unique within the list for the lifetime of the object that implements the WindowTimers interface.

The setTimeout() method must return the value returned by the timer initialization steps, passing them the method's arguments, the object on which the method for which the algorithm is running is implemented (a Window or WorkerGlobalScope object) as the method context, and the repeat flag set to false.

The setInterval() method must return the value returned by the timer initialization steps, passing them the method's arguments, the object on which the method for which the algorithm is running is implemented (a Window or WorkerGlobalScope object) as the method context, and the repeat flag set to true.

The clearTimeout() and clearInterval() methods must clear the entry identified as handle from the list of active timers of the WindowTimers object on which the method was invoked, where handle is the argument passed to the method, if any. (If handle does not identify an entry in the list of active timers of the WindowTimers object on which the method was invoked, the method does nothing.)

The timer initialization steps, which are invoked with some method arguments, a method context, a repeat flag which can be true or false, and optionally (and only if the repeat flag is true) a previous handle, are as follows:

1. Let method context proxy be method context if that is a WorkerGlobalScope object, or else the WindowProxy that corresponds to method context.

2. If previous handle was provided, let handle be previous handle; otherwise, let handle be a user-agent-defined integer that is greater than zero that will identify the timeout to be set by this call in the list of active timers.

3. If previous handle was not provided, add an entry to the list of active timers for handle.

4. Let task be a task that runs the following substeps:

   1. If the entry for handle in the list of active timers has been cleared, then abort this task's substeps.

   2. Run the appropriate set of steps from the following list:

      If the first method argument is a Function

      Call the Function. Use the third and subsequent method arguments (if any) as the arguments for invoking the Function. Use method context proxy as the thisArg for invoking the Function. [ECMA262]

      Otherwise

      1. Let script source be the first method argument.

      2. Let script language be JavaScript.

      3. Let settings object be method context's script settings object.

      4. Create a script using script source as the script source, the URL where script source can be found, scripting language as the scripting language, and settings object as the script settings object.

   3. If the repeat flag is true, then call timer initialization steps again, passing them the same method arguments, the same method context, with the repeat flag still set to true, and with the previous handle set to handler.

5. Let timeout be the second method argument, or zero if the argument was omitted.

6. If the currently running task is a task that was created by this algorithm, then let nesting level be the task's timer nesting level. Otherwise, let nesting level be zero.

7. If nesting level is greater than 5, and timeout is less than 4, then increase timeout to 4.

8. Increment nesting level by one.

9. Let task's timer nesting level be nesting level.

10. Return handle, and then continue running this algorithm asynchronously.

11. If method context is a Window object, wait until the Document associated with method context has been fully active for a further timeout milliseconds (not necessarily consecutively).

    Otherwise, method context is a WorkerGlobalScope object; wait until timeout milliseconds have passed with the worker not suspended (not necessarily consecutively).

12. Wait until any invocations of this algorithm that had the same method context, that started before this one, and whose timeout is equal to or less than this one's, have completed.

    Argument conversion as defined by Web IDL (for example, invoking toString() methods on objects passed as the first argument) happens in the algorithms defined in Web IDL, before this algorithm is invoked.

    So for example, the following rather silly code will result in the log containing "ONE TWO ":

    var log = '';
    function logger(s) { log += s + ' '; }
    
    setTimeout({ toString: function () {
      setTimeout("logger('ONE')", 100);
      return "logger('TWO')";
    } }, 100);

13. Optionally, wait a further user-agent defined length of time.

    This is intended to allow user agents to pad timeouts as needed to optimise the power usage of the device. For example, some processors have a low-power mode where the granularity of timers is reduced; on such platforms, user agents can slow timers down to fit this schedule instead of requiring the processor to use the more accurate mode with its associated higher power usage.

14. Queue the task task.

    Once the task has been processed, if the repeat flag is false, it is safe to remove the entry for handle from the list of active timers (there is no way for the entry's existence to be detected past this point, so it does not technically matter one way or the other).

The task source for these tasks is the timer task source.

The alert(message) method, when invoked, must run the following steps:

1. If the event loop's termination nesting level is non-zero, optionally abort these steps.

2. Release the storage mutex.

3. Optionally, abort these steps. (For example, the user agent might give the user the option to ignore all alerts, and would thus abort at this step whenever the method was invoked.)

4. Show the given message to the user.

5. Optionally, pause while waiting for the user to acknowledge the message.

The confirm(message) method, when invoked, must run the following steps:

1. If the event loop's termination nesting level is non-zero, optionally abort these steps, returning false.

2. Release the storage mutex.

3. Optionally, return false and abort these steps. (For example, the user agent might give the user the option to ignore all prompts, and would thus abort at this step whenever the method was invoked.)

4. Show the given message to the user, and ask the user to respond with a positive or negative response.

5. Pause until the user responds either positively or negatively.

6. If the user responded positively, return true; otherwise, the user responded negatively: return false.

The prompt(message, default) method, when invoked, must run the following steps:

1. If the event loop's termination nesting level is non-zero, optionally abort these steps, returning null.

2. Release the storage mutex.

3. Optionally, return null and abort these steps. (For example, the user agent might give the user the option to ignore all prompts, and would thus abort at this step whenever the method was invoked.)

4. Show the given message to the user, and ask the user to either respond with a string value or abort. The response must be defaulted to the value given by default.

5. Pause while waiting for the user's response.

6. If the user aborts, then return null; otherwise, return the string that the user responded with.

When the print() method is invoked, if the Document is ready for post-load tasks, then the user agent must synchronously run the printing steps. Otherwise, the user agent must only set the print when loaded flag on the Document.

User agents should also run the printing steps whenever the user asks for the opportunity to obtain a physical form (e.g. printed copy), or the representation of a physical form (e.g. PDF copy), of a document.

The printing steps are as follows:

1. The user agent may display a message to the user or abort these steps (or both).

   For instance, a kiosk browser could silently ignore any invocations of the print() method.

   For instance, a browser on a mobile device could detect that there are no printers in the vicinity and display a message saying so before continuing to offer a "save to PDF" option.

2. The user agent must fire a simple event named beforeprint at the Window object of the Document that is being printed, as well as any nested browsing contexts in it.

   The beforeprint event can be used to annotate the printed copy, for instance adding the time at which the document was printed.

3. The user agent must release the storage mutex.

4. The user agent should offer the user the opportunity to obtain a physical form (or the representation of a physical form) of the document. The user agent may wait for the user to either accept or decline before returning; if so, the user agent must pause while the method is waiting. Even if the user agent doesn't wait at this point, the user agent must use the state of the relevant documents as they are at this point in the algorithm if and when it eventually creates the alternate form.

5. The user agent must fire a simple event named afterprint at the Window object of the Document that is being printed, as well as any nested browsing contexts in it.

   The afterprint event can be used to revert annotations added in the earlier event, as well as showing post-printing UI. For instance, if a page is walking the user through the steps of applying for a home loan, the script could automatically advance to the next step after having printed a form or other.

The navigator attribute of the Window interface must return an instance of the Navigator interface, which represents the identity and state of the user agent (the client), and allows Web pages to register themselves as potential protocol and content handlers:

These interfaces are defined separately so that other specifications can re-use parts of the Navigator interface.

appCodeName

Must return the string "Mozilla".

appName

Must return either the string "Netscape" or the full name of the browser, e.g. "Mellblom Browsernator".

appVersion

Must return either the string "4.0" or a string representing the version of the browser in detail, e.g. "1.0 (VMS; en-US) Mellblomenator/9000".

platform

Must return either the empty string or a string representing the platform on which the browser is executing, e.g. "MacIntel", "Win32", "FreeBSD i386", "WebTV OS".

product

Must return the string "Gecko".

taintEnabled()

Must return false.

userAgent

Must return the string used for the value of the "User-Agent" header in HTTP requests, or the empty string if no such header is ever sent.

Any information in this API that varies from user to user can be used to profile the user. In fact, if enough such information is available, a user can actually be uniquely identified. For this reason, user agent implementors are strongly urged to include as little information in this API as possible.

language

Must return a valid BCP 47 language tag representing either a plausible language or the user's preferred language. [BCP47]

To determine a plausible language, the user agent should bear in mind the following:

• Any information in this API that varies from user to user can be used to profile or identify the user.
• If the user is not using a service that obfuscates the user's point of origin (e.g. the Tor anonymity network), then the value that is least likely to distinguish the user from other users with similar origins (e.g. from the same IP address block) is the language used by the majority of such users. [TOR]
• If the user is using an anonymizing service, then the value "en-US" is suggested; if all users of the service use that same value, that reduces the possibility of distinguishing the users from each other.

User agents may, within the constraints described in this section, do whatever they like when the methods are called. A UA could, for instance, prompt the user and offer the user the opportunity to add the site to a shortlist of handlers, or make the handlers his default, or cancel the request. UAs could provide such a UI through modal UI or through a non-modal transient notification interface. UAs could also simply silently collect the information, providing it only when relevant to the user.

User agents should keep track of which sites have registered handlers (even if the user has declined such registrations) so that the user is not repeatedly prompted with the same request.

The arguments to the methods have the following meanings and corresponding implementation requirements. The requirements that involve throwing exceptions must be processed in the order given below, stopping at the first exception thrown. (So the exceptions for the first argument take precedence over the exceptions for the second argument.)

scheme (registerProtocolHandler() only)

A scheme, such as mailto or web+auth. The scheme must be compared in an ASCII case-insensitive manner by user agents for the purposes of comparing with the scheme part of URLs that they consider against the list of registered handlers.

The scheme value, if it contains a colon (as in "mailto:"), will never match anything, since schemes don't contain colons.

If the registerProtocolHandler() method is invoked with a scheme that is neither a whitelisted scheme nor a scheme whose value starts with the substring "web+" and otherwise contains only lowercase ASCII letters, and whose length is at least five characters (including the "web+" prefix), the user agent must throw a SecurityError exception.

The following schemes are the whitelisted schemes:

• bitcoin
• geo
• im
• irc
• ircs
• magnet
• mailto
• mms
• news
• nntp
• sip
• sms
• smsto
• ssh
• tel
• urn
• webcal
• wtai
• xmpp

This list can be changed. If there are schemes that should be added, please send feedback.

This list excludes any schemes that could reasonably be expected to be supported inline, e.g. in an iframe, such as http or (more theoretically) gopher. If those were supported, they could potentially be used in man-in-the-middle attacks, by replacing pages that have frames with such content with content under the control of the protocol handler. If the user agent has native support for the schemes, this could further be used for cookie-theft attacks.

mimeType (registerContentHandler() only)

A MIME type, such as model/vnd.flatland.3dml or application/vnd.google-earth.kml+xml. The MIME type must be compared in an ASCII case-insensitive manner by user agents for the purposes of comparing with MIME types of documents that they consider against the list of registered handlers.

User agents must compare the given values only to the MIME type/subtype parts of content types, not to the complete type including parameters. Thus, if mimeType values passed to this method include characters such as commas or whitespace, or include MIME parameters, then the handler being registered will never be used.

The type is compared to the MIME type used by the user agent after the sniffing algorithms have been applied.

If the registerContentHandler() method is invoked with a MIME type that is in the type blacklist or that the user agent has deemed a privileged type, the user agent must throw a SecurityError exception.

The following MIME types are in the type blacklist:

• application/x-www-form-urlencoded
• application/xhtml+xml
• application/xml
• image/gif
• image/jpeg
• image/png
• image/svg+xml
• multipart/x-mixed-replace
• text/cache-manifest
• text/css
• text/html
• text/ping
• text/plain
• text/xml
• All types that the user agent supports displaying natively in a browsing context during navigation, except for application/rss+xml and application/atom+xml

This list can be changed. If there are MIME types that should be added, please send feedback.

url

A string used to build the URL of the page that will handle the requests.

User agents must throw a SyntaxError exception if the url argument passed to one of these methods does not contain the exact literal string "%s".

User agents must throw a SyntaxError exception if resolving the url argument relative to the API base URL specified by the entry settings object is not successful.

The resulting absolute URL would by definition not be a valid URL as it would include the string "%s" which is not a valid component in a URL.

User agents must throw a SecurityError exception if the resulting absolute URL has an origin that differs from the origin specified by the entry settings object.

This is forcibly the case if the %s placeholder is in the scheme, host, or port parts of the URL.

The resulting absolute URL is the proto-URL. It identifies the handler for the purposes of the methods described below.

When the user agent uses this handler, it must replace the first occurrence of the exact literal string "%s" in the url argument with an escaped version of the absolute URL of the content in question (as defined below), then resolve the resulting URL, relative to the API base URL specified by the entry settings object at the time the registerContentHandler() or registerProtocolHandler() methods were invoked, and then navigate an appropriate browsing context to the resulting URL using the GET method (or equivalent for non-HTTP URLs).

To get the escaped version of the absolute URL of the content in question, the user agent must replace every character in that absolute URL that is not a character in the URL default encode set with the result of UTF-8 percent encoding that character.

If the user had visited a site at http://example.com/ that made the following call:

navigator.registerContentHandler('application/x-soup', 'soup?url=%s', 'SoupWeb™')

...and then, much later, while visiting http://www.example.net/, clicked on a link such as:

<a href="chickenkïwi.soup">Download our Chicken Kïwi soup!</a>

...then, assuming this chickenkïwi.soup file was served with the MIME type application/x-soup, the UA might navigate to the following URL:

http://example.com/soup?url=http://www.example.net/chickenk%C3%AFwi.soup

This site could then fetch the chickenkïwi.soup file and do whatever it is that it does with soup (synthesize it and ship it to the user, or whatever).

title

A descriptive title of the handler, which the UA might use to remind the user what the site in question is.

This section does not define how the pages registered by these methods are used, beyond the requirements on how to process the url value (see above). To some extent, the processing model for navigating across documents defines some cases where these methods are relevant, but in general UAs may use this information wherever they would otherwise consider handing content to native plugins or helper applications.

UAs must not use registered content handlers to handle content that was returned as part of a non-GET transaction (or rather, as part of any non-idempotent transaction), as the remote site would not be able to fetch the same data.

The handler state strings are the following strings. Each string describes several situations, as given by the following list.

new

The described handler has never been registered for the given scheme or type.

The described handler was once registered for the given scheme or type, but the site has since unregistered it. If the handler were to be reregistered, the user would be notified accordingly.

The described handler was once registered for the given scheme or type, but the site has since unregistered it, but the user has indicated that the site is to be blocked from registering the type again, so the user agent would ignore further registration attempts.

registered

An attempt was made to register the described handler for the given scheme or type, but the user has not yet been notified, and the user agent would ignore further registration attempts. (Maybe the user agent batches registration requests to display them when the user requests to be notified about them, and the user has not yet requested that the user agent notify it of the previous registration attempt.)

The described handler is registered for the given scheme or type (maybe, or maybe not, as the default handler).

The described handler is permanently blocked from being (re)registered. (Maybe the user marked the registration attempt as spam, or blocked the site for other reasons.)

declined

An attempt was made to register the described handler for the given scheme or type, but the user has not yet been notified; however, the user might be notified if another registration attempt were to be made. (Maybe the last registration attempt was made while the page was in the background and the user closed the page without looking at it, and the user agent requires confirmation for this registration attempt.)

An attempt was made to register the described handler for the given scheme or type, but the user has not yet responded.

An attempt was made to register the described handler for the given scheme or type, but the user declined the offer. The user has not indicated that the handler is to be permanently blocked, however, so another attempt to register the described handler might result in the user being prompted again.

The described handler was once registered for the given scheme or type, but the user has since removed it. The user has not indicated that the handler is to be permanently blocked, however, so another attempt to register the described handler might result in the user being prompted again.

The unregisterProtocolHandler() method must unregister the handler described by the two arguments to the method, where the first argument gives the scheme and the second gives the string used to build the URL of the page that will handle the requests.

The first argument must be compared to the schemes for which custom protocol handlers are registered in an ASCII case-insensitive manner to find the relevant handlers.

The second argument must be preprocessed as described below, and if that is successful, must then be matched against the proto-URLs of the relevant handlers to find the described handler.

The unregisterContentHandler() method must unregister the handler described by the two arguments to the method, where the first argument gives the MIME type and the second gives the string used to build the URL of the page that will handle the requests.

The first argument must be compared to the MIME types for which custom content handlers are registered in an ASCII case-insensitive manner to find the relevant handlers.

The second argument must be preprocessed as described below, and if that is successful, must then be matched against the proto-URLs of the relevant handlers to find the described handler.

The second argument of the four methods described above must be preprocessed as follows:

1. If the string does not contain the substring "%s", abort these steps. There's no matching handler.

2. Resolve the string relative to the API base URL specified by the entry settings object.

3. If this fails, then throw a SyntaxError exception, aborting the method.

4. If the resulting absolute URL's origin is not the same origin as the origin specified by the entry settings object, throw a SecurityError exception, aborting the method.

5. Return the resulting absolute URL as the result of preprocessing the argument.

6.6.1.3.1 Security and privacy

These mechanisms can introduce a number of concerns, in particular privacy concerns.

Hijacking all Web usage. User agents should not allow schemes that are key to its normal operation, such as http or https, to be rerouted through third-party sites. This would allow a user's activities to be trivially tracked, and would allow user information, even in secure connections, to be collected.

Hijacking defaults. User agents are strongly urged to not automatically change any defaults, as this could lead the user to send data to remote hosts that the user is not expecting. New handlers registering themselves should never automatically cause those sites to be used.

Registration spamming. User agents should consider the possibility that a site will attempt to register a large number of handlers, possibly from multiple domains (e.g. by redirecting through a series of pages each on a different domain, and each registering a handler for video/mpeg — analogous practices abusing other Web browser features have been used by pornography Web sites for many years). User agents should gracefully handle such hostile attempts, protecting the user.

Misleading titles. User agents should not rely wholly on the title argument to the methods when presenting the registered handlers to the user, since sites could easily lie. For example, a site hostile.example.net could claim that it was registering the "Cuddly Bear Happy Content Handler". User agents should therefore use the handler's domain in any UI along with any title.

Hostile handler metadata. User agents should protect against typical attacks against strings embedded in their interface, for example ensuring that markup or escape characters in such strings are not executed, that null bytes are properly handled, that over-long strings do not cause crashes or buffer overruns, and so forth.

Leaking Intranet URLs. The mechanism described in this section can result in secret Intranet URLs being leaked, in the following manner:

1. The user registers a third-party content handler as the default handler for a content type.
2. The user then browses his corporate Intranet site and accesses a document that uses that content type.
3. The user agent contacts the third party and hands the third party the URL to the Intranet content.

No actual confidential file data is leaked in this manner, but the URLs themselves could contain confidential information. For example, the URL could be http://www.corp.example.com/upcoming-aquisitions/the-sample-company.egf, which might tell the third party that Example Corporation is intending to merge with The Sample Company. Implementors might wish to consider allowing administrators to disable this feature for certain subdomains, content types, or schemes.

Leaking secure URLs. User agents should not send HTTPS URLs to third-party sites registered as content handlers without the user's informed consent, for the same reason that user agents sometimes avoid sending Referer (sic) HTTP headers from secure sites to third-party sites.

Leaking credentials. User agents must never send username or password information in the URLs that are escaped and included sent to the handler sites. User agents may even avoid attempting to pass to Web-based handlers the URLs of resources that are known to require authentication to access, as such sites would be unable to access the resources in question without prompting the user for credentials themselves (a practice that would require the user to know whether to trust the third-party handler, a decision many users are unable to make or even understand).

Interface interference. User agents should be prepared to handle intentionally long arguments to the methods. For example, if the user interface exposed consists of an "accept" button and a "deny" button, with the "accept" binding containing the name of the handler, it's important that a long name not cause the "deny" button to be pushed off the screen.

Fingerprinting users. Since a site can detect if it has attempted to register a particular handler or not, whether or not the user responds, the mechanism can be used to store data. User agents are therefore strongly urged to treat registrations in the same manner as cookies: clearing cookies for a site should also clear all registrations for that site, and disabling cookies for a site should also disable registrations.

6.6.1.3.2 Sample user interface

This section is non-normative.

A simple implementation of this feature for a desktop Web browser might work as follows.

The registerContentHandler() method could display a modal dialog box:

In this dialog box, "Kittens at work" is the title of the page that invoked the method, "http://kittens.example.org/" is the URL of that page, "application/x-meowmeow" is the string that was passed to the registerContentHandler() method as its first argument (mimeType), "http://kittens.example.org/?show=%s" was the second argument (url), and "Kittens-at-work displayer" was the third argument (title).

If the user clicks the Cancel button, then nothing further happens. If the user clicks the "Trust" button, then the handler is remembered.

When the user then attempts to fetch a URL that uses the "application/x-meowmeow" MIME type, then it might display a dialog as follows:

In this dialog, the third option is the one that was primed by the site registering itself earlier.

If the user does select that option, then the browser, in accordance with the requirements described in the previous two sections, will redirect the user to "http://kittens.example.org/?show=data%3Aapplication/x-meowmeow;base64,S2l0dGVucyBhcmUgdGhlIGN1dGVzdCE%253D".

The registerProtocolHandler() method would work equivalently, but for schemes instead of unknown content types.

The cookieEnabled attribute must return true if the user agent attempts to handle cookies according to the cookie specification, and false if it ignores cookie change requests. [COOKIES]

The yieldForStorageUpdates() method, when invoked, must, if the storage mutex is owned by the event loop of the task that resulted in the method being called, release the storage mutex so that it is once again free. Otherwise, it must do nothing.

The navigator.plugins attribute must return a PluginArray object. The same object must be returned each time.

The navigator.mimeTypes attribute must return a MimeTypeArray object. The same object must be returned each time.

A PluginArray object represents none, some, or all of the plugins supported by the user agent, each of which is represented by a Plugin object. Each of these Plugin objects may be hidden plugins. A hidden plugin can't be enumerated, but can still be inspected by using its name.

The fewer plugins are represented by the PluginArray object, and of those, the more that are hidden, the more the user's privacy will be protected. Each exposed plugin increases the number of bits that can be derived for fingerprinting. Hiding a plugin helps, but unless it is an extremely rare plugin, it is likely that a site attempting to derive the list of plugins can still determine whether the plugin is supported or not by probing for it by name (the names of popular plugins are widely known). Therefore not exposing a plugin at all is preferred. Unfortunately, many legacy sites use this feature to determine, for example, which plugin to use to play video. Not exposing any plugins at all might therefore not be entirely plausible.

The PluginArray objects created by a user agent must not be live. The set of plugins represented by the objects must not change once an object is created, except when it is updated by the refresh() method.

Each plugin represented by a PluginArray can support a number of MIME types. For each such plugin, the user agent must pick one or more of these MIME types to be those that are explicitly supported.

The explicitly supported MIME types of a plugin are those that are exposed through the Plugin and MimeTypeArray interfaces. As with plugins themselves, any variation between users regarding what is exposed allows sites to fingerprint users. User agents are therefore encouraged to expose the same MIME types for all users of a plugin, regardless of the actual types supported... at least, within the constraints imposed by compatibility with legacy content.

The supported property indices of a PluginArray object are the numbers from zero to the number of non-hidden plugins represented by the object, if any.

The length attribute must return the number of non-hidden plugins represented by the object.

The item() method of a PluginArray object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

1. Let list be the Plugin objects representing the non-hidden plugins represented by the PluginArray object.

2. Sort list alphabetically by the name of each Plugin.

3. Return the indexth entry in list.

It is important for privacy that the order of plugins not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a PluginArray object are the values of the name attributes of all the Plugin objects represented by the PluginArray object. The properties exposed in this way must not be enumerable.

The namedItem() method of a PluginArray object must return null if the argument is not one of the object's supported property names, and otherwise must return the Plugin object, of those represented by the PluginArray object, that has a name equal to the method's argument.

The refresh() method of the PluginArray object of a Navigator object, when invoked, must check to see if any plugins have been installed or reconfigured since the user agent created the PluginArray object. If so, and the method's argument is true, then the user agent must act as if the location.reload() method was called instead. Otherwise, the user agent must update the PluginArray object and MimeTypeArray object created for attributes of that Navigator object, and the Plugin and MimeType objects created for those PluginArray and MimeTypeArray objects, using the same Plugin objects for cases where the name is the same, and the same MimeType objects for cases where the type is the same, and creating new objects for cases where there were no matching objects immediately prior to the refresh() call. Old Plugin and MimeType objects must continue to return the same values that they had prior to the update, though naturally now the data is stale and may appear inconsistent (for example, an old MimeType entry might list as its enabledPlugin a Plugin object that no longer lists that MimeType as a supported MimeType).

A MimeTypeArray object represents the MIME types explicitly supported by plugins supported by the user agent, each of which is represented by a MimeType object.

The MimeTypeArray objects created by a user agent must not be live. The set of MIME types represented by the objects must not change once an object is created, except when it is updated by the PluginArray object's refresh() method.

The supported property indices of a MimeTypeArray object are the numbers from zero to the number of MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.

The length attribute must return the number of MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.

The item() method of a MimeTypeArray object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

1. Let list be the MimeType objects representing the MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.

2. Sort list alphabetically by the type of each MimeType.

3. Return the indexth entry in list.

It is important for privacy that the order of MIME types not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a MimeTypeArray object are the values of the type attributes of all the MimeType objects represented by the MimeTypeArray object. The properties exposed in this way must not be enumerable.

The namedItem() method of a MimeTypeArray object must return null if the argument is not one of the object's supported property names, and otherwise must return the MimeType object that has a type equal to the method's argument.

A Plugin object represents a plugin. It has several attributes to provide details about the plugin, and can be enumerated to obtain the list of MIME types that it explicitly supports.

The Plugin objects created by a user agent must not be live. The set of MIME types represented by the objects, and the values of the objects' attributes, must not change once an object is created, except when updated by the PluginArray object's refresh() method.

The reported MIME types for a Plugin object are the MIME types explicitly supported by the corresponding plugin when this object was last created or updated by PluginArray.refresh(), whichever happened most recently.

The supported property indices of a Plugin object are the numbers from zero to the number of reported MIME types.

The length attribute must return the number of reported MIME types.

The item() method of a Plugin object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

1. Let list be the MimeType objects representing the reported MIME types.

2. Sort list alphabetically by the type of each MimeType.

3. Return the indexth entry in list.

It is important for privacy that the order of MIME types not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a Plugin object are the values of the type attributes of the MimeType objects representing the reported MIME types. The properties exposed in this way must not be enumerable.

The namedItem() method of a Plugin object must return null if the argument is not one of the object's supported property names, and otherwise must return the MimeType object that has a type equal to the method's argument.

The name attribute must return the plugin's name.

The description and filename attributes must return user-agent-defined (or, in all likelihood, plugin-defined) strings. In each case, the same string must be returned each time, except that the strings returned may change when the PluginArray.refresh() method updates the object.

If the values returned by the description or filename attributes vary between versions of a plugin, they can be used both as a fingerprinting vector and, even more importantly, as a trivial way to determine what security vulnerabilities a plugin (and thus a browser) may have. It is thus highly recommended that the description attribute just return the same value as the name attribute, and that the filename attribute return the empty string.

A MimeType object represents a MIME type that is, or was, explicitly supported by a plugin.

The MimeType objects created by a user agent must not be live. The values of the objects' attributes must not change once an object is created, except when updated by the PluginArray object's refresh() method.

The type attribute must return the valid MIME type with no parameters describing the MIME type.

The description and suffixes attributes must return user-agent-defined (or, in all likelihood, plugin-defined) strings. In each case, the same string must be returned each time, except that the strings returned may change when the PluginArray.refresh() method updates the object.

If the values returned by the description or suffxies attributes vary between versions of a plugin, they can be used both as a fingerprinting vector and, even more importantly, as a trivial way to determine what security vulnerabilities a plugin (and thus a browser) may have. It is thus highly recommended that the description attribute just return the same value as the type attribute, and that the suffixes attribute return the empty string.

Commas in the suffixes attribute are interpreted as separating subsequent filename extensions, as in "htm,html".

The enabledPlugin attribute must return the Plugin object that represents the plugin that explicitly supported the MIME type that this MimeType object represents when this object was last created or updated by PluginArray.refresh(), whichever happened most recently.

The navigator.javaEnabled attribute must return true if the user agent supports a plugin that supports the MIME type "application/x-java-vm".

The external attribute of the Window interface must return an instance of the External interface. The same object must be returned each time.

The AddSearchProvider() method, when invoked, must run the following steps:

1. Optionally, abort these steps. User agents may implement the method as a stub method that never does anything, or may arbitrarily ignore invocations with particular arguments for security, privacy, or usability reasons.

2. Resolve the value of the method's first argument relative to the API base URL specified by the entry settings object.

3. If this fails, abort these steps.

4. Process the resulting absolute URL as the URL to an OpenSearch description document. [OPENSEARCH]

The IsSearchProviderInstalled() method, when invoked, must run the following steps:

1. Optionally, return 0 and abort these steps. User agents may implement the method as a stub method that never returns a non-zero value, or may arbitrarily ignore invocations with particular arguments for security, privacy, or usability reasons.

2. If the origin specified by the entry settings object is an opaque identifier (i.e. it has no host component), then return 0 and abort these steps.

3. Let host1 be the host component of the origin specified by the entry settings object.

4. Resolve the scriptURL argument relative to the API base URL specified by the entry settings object.

5. If this fails, return 0 and abort these steps.

6. Let host2 be the host component of the resulting parsed URL.

7. If the longest suffix in the Public Suffix List that matches the end of host1 is different than the longest suffix in the Public Suffix List that matches the end of host2, then return 0 and abort these steps. [PSL]

   If the next domain component of host1 and host2 after their common suffix are not the same, then return 0 and abort these steps.

8. Let search engines be the list of search engines known by the user agent and made available to the user by the user agent for which the resulting absolute URL is a prefix match of the search engine's URL, if any. For search engines registered using OpenSearch description documents, the URL of the search engine corresponds to the URL given in a Url element whose rel attribute is "results" (the default). [OPENSEARCH]

9. If search engines is empty, return 0 and abort these steps.

10. If the user's default search engine (as determined by the user agent) is one of the search engines in search engines, then return 2 and abort these steps.

11. Return 1.