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   <p><a href="http://www.w3.org/"><img alt="W3C" height="48" src="http://www.w3.org/Icons/w3c_home" width="72"></a></p>

   <h1>HTML5</h1>
   </div><div>
   <a href="webappapis.html" class="prev">6 Web application APIs</a> &#8211;
   <a href="spec.html#contents">Table of contents</a> &#8211;
   <a href="editing.html" class="next">7 User interaction</a>
  <ol class="toc"><li><ol><li><a href="timers.html#timers"><span class="secno">6.3 </span>Timers</a></li><li><a href="timers.html#user-prompts"><span class="secno">6.4 </span>User prompts</a>
    <ol><li><a href="timers.html#simple-dialogs"><span class="secno">6.4.1 </span>Simple dialogs</a></li><li><a href="timers.html#printing"><span class="secno">6.4.2 </span>Printing</a></li><li><a href="timers.html#dialogs-implemented-using-separate-documents"><span class="secno">6.4.3 </span>Dialogs implemented using separate documents</a></li></ol></li><li><a href="timers.html#system-state-and-capabilities:-the-navigator-object"><span class="secno">6.5 </span>System state and capabilities: the <code>Navigator</code> object</a>
    <ol><li><a href="timers.html#client-identification"><span class="secno">6.5.1 </span>Client identification</a></li><li><a href="timers.html#custom-handlers"><span class="secno">6.5.2 </span>Custom scheme and content handlers</a>
      <ol><li><a href="timers.html#security-and-privacy"><span class="secno">6.5.2.1 </span>Security and privacy</a></li><li><a href="timers.html#sample-handler-impl"><span class="secno">6.5.2.2 </span>Sample user interface</a></li></ol></li><li><a href="timers.html#manually-releasing-the-storage-mutex"><span class="secno">6.5.3 </span>Manually releasing the storage mutex</a></li></ol></li></ol></li></ol></div>

  <h3 id="timers"><span class="secno">6.3 </span>Timers</h3><p>The <code title="dom-windowtimers-setTimeout"><a href="#dom-windowtimers-settimeout">setTimeout()</a></code>
  and <code title="dom-windowtimers-setInterval"><a href="#dom-windowtimers-setinterval">setInterval()</a></code>
  methods allow authors to schedule timer-based callbacks.</p><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="windowtimers">WindowTimers</dfn> {
  long <a href="#dom-windowtimers-settimeout" title="dom-windowtimers-setTimeout">setTimeout</a>(in any handler, in optional any timeout, in any... args);
  void <a href="#dom-windowtimers-cleartimeout" title="dom-windowtimers-clearTimeout">clearTimeout</a>(in long handle);
  long <a href="#dom-windowtimers-setinterval" title="dom-windowtimers-setInterval">setInterval</a>(in any handler, in optional any timeout, in any... args);
  void <a href="#dom-windowtimers-clearinterval" title="dom-windowtimers-clearInterval">clearInterval</a>(in long handle);
};
<a href="browsers.html#window">Window</a> implements <a href="#windowtimers">WindowTimers</a>;</pre><dl class="domintro"><dt><var title="">handle</var> = <var title="">window</var> . <code title="dom-windowtimers-setTimeout"><a href="#dom-windowtimers-settimeout">setTimeout</a></code>( <var title="">handler</var> [, <var title="">timeout</var> [, <var title="">arguments</var> ] ] )</dt>

   <dd>

    <p>Schedules a timeout to run <var title="">handler</var> after
    <var title="">timeout</var> milliseconds. Any <var title="">arguments</var> are passed straight through to the <var title="">handler</var>.</p>

   </dd>

   <dt><var title="">handle</var> = <var title="">window</var> . <code title="dom-windowtimers-setTimeout"><a href="#dom-windowtimers-settimeout">setTimeout</a></code>( <var title="">code</var> [, <var title="">timeout</var> ] )</dt>

   <dd>

    <p>Schedules a timeout to compile and run <var title="">code</var>
    after <var title="">timeout</var> milliseconds.</p>

   </dd>

   <dt><var title="">window</var> . <code title="dom-windowtimers-clearTimeout"><a href="#dom-windowtimers-cleartimeout">clearTimeout</a></code>( <var title="">handle</var> )</dt>

   <dd>

    <p>Cancels the timeout set with <code title="dom-windowtimers-setTimeout"><a href="#dom-windowtimers-settimeout">setTimeout()</a></code> identified by <var title="">handle</var>.</p>

   </dd>

   <dt><var title="">handle</var> = <var title="">window</var> . <code title="dom-windowtimers-setInterval"><a href="#dom-windowtimers-setinterval">setInterval</a></code>( <var title="">handler</var> [, <var title="">timeout</var> [, <var title="">arguments</var> ] ] )</dt>

   <dd>

    <p>Schedules a timeout to run <var title="">handler</var> every
    <var title="">timeout</var> milliseconds. Any <var title="">arguments</var> are passed straight through to the <var title="">handler</var>.</p>

   </dd>

   <dt><var title="">handle</var> = <var title="">window</var> . <code title="dom-windowtimers-setInterval"><a href="#dom-windowtimers-setinterval">setInterval</a></code>( <var title="">code</var> [, <var title="">timeout</var> ] )</dt>

   <dd>

    <p>Schedules a timeout to compile and run <var title="">code</var>
    every <var title="">timeout</var> milliseconds.</p>

   </dd>

   <dt><var title="">window</var> . <code title="dom-windowtimers-clearInterval"><a href="#dom-windowtimers-clearinterval">clearInterval</a></code>( <var title="">handle</var> )</dt>

   <dd>

    <p>Cancels the timeout set with <code title="dom-windowtimers-setInterval"><a href="#dom-windowtimers-setinterval">setInterval()</a></code> identified by <var title="">handle</var>.</p>

   </dd>

  </dl><p class="note">This API does not guarantee that timers will fire
  exactly on schedule. Delays due to CPU load, other tasks, etc, are
  to be expected.</p><div class="impl">

  <p class="note">The <code><a href="#windowtimers">WindowTimers</a></code> interface adds to the
  <code><a href="browsers.html#window">Window</a></code> interface and the <code>WorkerUtils</code>
  interface (part of Web Workers).</p>

  <p>Each object that implements the <code><a href="#windowtimers">WindowTimers</a></code>
  interface has a <dfn id="list-of-active-timeouts">list of active timeouts</dfn> and a <dfn id="list-of-active-intervals">list
  of active intervals</dfn>. Each entry in these lists is identified
  by a number, which must be unique within its list for the lifetime
  of the object that implements the <code><a href="#windowtimers">WindowTimers</a></code>
  interface.</p>

  <hr><p>The <dfn id="dom-windowtimers-settimeout" title="dom-windowtimers-setTimeout"><code>setTimeout()</code></dfn>
  method must run the following steps:

  </p><ol><li><p>Let <var title="">handle</var> be a user-agent-defined integer
   that is greater than zero that will identify the timeout to be set
   by this call.</p></li>

   <li><p>Add an entry to the <a href="#list-of-active-timeouts">list of active timeouts</a> for
   <var title="">handle</var>.</p></li>

   <li><p><a href="#get-the-timed-task">Get the timed task</a> <var title="">handle</var> in
   the <a href="#list-of-active-timeouts">list of active timeouts</a>, and let <var title="">task</var> be the result.</p></li>

   <li><p><a href="#get-the-timeout">Get the timeout</a>, and let <var title="">timeout</var> be the result.</p></li>

   <li><p>If the currently running <a href="webappapis.html#concept-task" title="concept-task">task</a> is a task that was created by the
   <code title="dom-windowtimers-setTimeout"><a href="#dom-windowtimers-settimeout">setTimeout()</a></code>
   method, and <var title="">timeout</var> is less than 4, then
   increase <var title="">timeout</var> to 4.</p></li>

   <li><p>Return <var title="">handle</var>, and then continue running
   this algorithm asynchronously.</p></li>

   <li>

    <p>If the <a href="#method-context">method context</a> is a <code><a href="browsers.html#window">Window</a></code>
    object, wait until the <code><a href="infrastructure.html#document">Document</a></code> associated with the
    <a href="#method-context">method context</a> has been <a href="browsers.html#fully-active">fully active</a> for
    a further <var title="">timeout</var> milliseconds (not
    necessarily consecutively).</p>

    <p>Otherwise, if the <a href="#method-context">method context</a> is a
    <code>WorkerUtils</code> object, wait until <var title="">timeout</var> milliseconds have passed with the worker
    not suspended (not necessarily consecutively).</p>

    <p>Otherwise, act as described in the specification that defines
    that the <code><a href="#windowtimers">WindowTimers</a></code> interface is implemented by
    some other object.</p>

   </li>

   <li><p>Wait until any invocations of this algorithm started before
   this one whose <var title="">timeout</var> is equal to or less than
   this one's have completed.</p></li>

   <li>

    <p>Optionally, wait a further user-agent defined length of
    time.</p>

    <p class="note">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.</p>

   </li>

   <li><p><a href="webappapis.html#queue-a-task" title="queue a task">Queue</a> the <var title="">task</var> <a href="webappapis.html#concept-task" title="concept-task">task</a>.</p></li>

  </ol><p>The <dfn id="dom-windowtimers-cleartimeout" title="dom-windowtimers-clearTimeout"><code>clearTimeout()</code></dfn>
  method must clear the entry identified as <var title="">handle</var>
  from the <a href="#list-of-active-timeouts">list of active timeouts</a> of the
  <code><a href="#windowtimers">WindowTimers</a></code> object on which the method was invoked,
  where <var title="">handle</var> is the argument passed to the
  method, if any. (If <var title="">handle</var> does not identify an
  entry in the <a href="#list-of-active-timeouts">list of active timeouts</a> of the
  <code><a href="#windowtimers">WindowTimers</a></code> object on which the method was invoked,
  the method does nothing.)</p>

  <hr><p>The <dfn id="dom-windowtimers-setinterval" title="dom-windowtimers-setInterval"><code>setInterval()</code></dfn>
  method must run the following steps:

  </p><ol><li><p>Let <var title="">handle</var> be a user-agent-defined integer
   that is greater than zero that will identify the interval to be set
   by this call.</p></li>

   <li><p>Add an entry to the <a href="#list-of-active-intervals">list of active intervals</a> for
   <var title="">handle</var>.</p></li>

   <li><p><a href="#get-the-timed-task">Get the timed task</a> <var title="">handle</var> in
   the <a href="#list-of-active-intervals">list of active intervals</a>, and let <var title="">task</var> be the result.</p></li>

   <li><p><a href="#get-the-timeout">Get the timeout</a>, and let <var title="">timeout</var> be the result.</p></li>

   <li><p>If <var title="">timeout</var> is less than 10, then
   increase <var title="">timeout</var> to 10.</p></li> 
   <li><p>Return <var title="">handle</var>, and then continue running
   this algorithm asynchronously.</p></li>

   
   <li>

    <p><i title="">Wait</i>: If the <a href="#method-context">method context</a> is a
    <code><a href="browsers.html#window">Window</a></code> object, wait until the <code><a href="infrastructure.html#document">Document</a></code>
    associated with the <a href="#method-context">method context</a> has been <a href="browsers.html#fully-active">fully
    active</a> for a further <var title="">interval</var>
    milliseconds (not necessarily consecutively).</p>

    <p>Otherwise, if the <a href="#method-context">method context</a> is a
    <code>WorkerUtils</code> object, wait until <var title="">interval</var> milliseconds have passed with the worker
    not suspended (not necessarily consecutively).</p>

    <p>Otherwise, act as described in the specification that defines
    that the <code><a href="#windowtimers">WindowTimers</a></code> interface is implemented by
    some other object.</p>

   </li>

   <li>

    <p>Optionally, wait a further user-agent defined length of
    time.</p>

    <p class="note">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.</p>

   </li>

   <li><p><a href="webappapis.html#queue-a-task" title="queue a task">Queue</a> the <var title="">task</var> <a href="webappapis.html#concept-task" title="concept-task">task</a>.</p></li>

   <li><p>Return to the step labeled <i>wait</i>.</p></li>

  </ol><p>The <dfn id="dom-windowtimers-clearinterval" title="dom-windowtimers-clearInterval"><code>clearInterval()</code></dfn>
  method must clear the entry identified as <var title="">handle</var>
  from the <a href="#list-of-active-intervals">list of active intervals</a> of the
  <code><a href="#windowtimers">WindowTimers</a></code> object on which the method was invoked,
  where <var title="">handle</var> is the argument passed to the
  method, if any. (If <var title="">handle</var> does not identify an
  entry in the <a href="#list-of-active-intervals">list of active intervals</a> of the
  <code><a href="#windowtimers">WindowTimers</a></code> object on which the method was invoked,
  the method does nothing.)</p>

  <hr><p>The <dfn id="method-context">method context</dfn>, when referenced by the algorithms
  in this section, is the object on which the method for which the
  algorithm is running is implemented (a <code><a href="browsers.html#window">Window</a></code> or
  <code>WorkerUtils</code> object).</p>

  <p>When the above methods are invoked and try to <dfn id="get-the-timed-task">get the timed
  task</dfn> <var title="">handle</var> in list <var title="">list</var>,
  they must run the following steps:</p>

  <ol><li>

    <p>If the first argument to the invoked method is an object that
    has an internal [[Call]] method, then return a <a href="webappapis.html#concept-task" title="concept-task">task</a> that checks if the entry for <var title="">handle</var> in <var title="">list</var> has been
    cleared, and if it has not, calls the aforementioned [[Call]]
    method with as its arguments the third and subsequent arguments to
    the invoked method (if any), and with an undefined <var title="">thisArg</var>, and abort these steps. <a href="references.html#refsECMA262">[ECMA262]</a></p>

    <p class="note">Setting <var title="">thisArg</var> to undefined
    means that the function code will be executed with the <code title="">this</code> keyword bound to the <code><a href="browsers.html#windowproxy">WindowProxy</a></code>
    or the <code>WorkerGlobalScope</code> object, as if the code was
    running in the global scope.</p>

    <p>Otherwise, continue with the remaining steps.</p>

   </li>

   <li><p>Apply the ToString() abstract operation to the first
   argument to the method, and let <var title="">script source</var>
   be the result. <a href="references.html#refsECMA262">[ECMA262]</a></p></li>

   <li><p>Let <var title="">script language</var> be
   JavaScript.</p></li>

   <li>

    <p>If the <a href="#method-context">method context</a> is a <code><a href="browsers.html#window">Window</a></code>
    object, let <var title="">global object</var> be the <a href="#method-context">method
    context</a>, let <var title="">browsing context</var> be the
    <a href="browsers.html#browsing-context">browsing context</a> with which <var title="">global
    object</var> is associated, let <var title="">character
    encoding</var> be the <a href="dom.html#document-s-character-encoding" title="document's character
    encoding">character encoding</a> of the <code><a href="infrastructure.html#document">Document</a></code>
    associated with <var title="">global object</var> (<a href="webappapis.html#sce-not-copy">this is a reference, not a copy</a>), and let
    <var title="">base URL</var> be the <a href="urls.html#document-base-url" title="document base
    URL">base URL</a> of the <code><a href="infrastructure.html#document">Document</a></code> associated with
    <var title="">global object</var> (<a href="webappapis.html#sbu-not-copy">this is
    a reference, not a copy</a>).</p>

    <p>Otherwise, if the <a href="#method-context">method context</a> is a
    <code>WorkerUtils</code> object, let <var title="">global
    object</var>, <var title="">browsing context</var>, <var title="">document</var>, <var title="">character encoding</var>,
    and <var title="">base URL</var> be the <a href="webappapis.html#script-s-global-object">script's global
    object</a>, <a href="webappapis.html#script-s-browsing-context">script's browsing context</a>,
    <a href="webappapis.html#script-s-document">script's document</a>, <a href="webappapis.html#script-s-url-character-encoding">script's URL character
    encoding</a>, and <a href="webappapis.html#script-s-base-url">script's base URL</a> (respectively)
    of the <a href="webappapis.html#concept-script" title="concept-script">script</a> that the
    <span>run a worker</span> algorithm created when it created the
    <a href="#method-context">method context</a>.</p>

    <p>Otherwise, act as described in the specification that defines
    that the <code><a href="#windowtimers">WindowTimers</a></code> interface is implemented by
    some other object.</p>

   </li>

   <li><p>Return a <a href="webappapis.html#concept-task" title="concept-task">task</a> that checks
   if the entry for <var title="">handle</var> in <var title="">list</var> has been cleared, and if it has not, <a href="webappapis.html#create-a-script" title="create a script">creates a script</a> using <var title="">script source</var> as the script source, <var title="">scripting language</var> as the scripting language, <var title="">global object</var> as the global object, <var title="">browsing context</var> as the browsing context, <var title="">document</var> as the document, <var title="">character
   encoding</var> as the URL character encoding, and <var title="">base URL</var> as the base URL.</p></li>

  </ol><p>When the above methods are to <dfn id="get-the-timeout">get the timeout</dfn>, they
  must run the following steps:</p>

  <ol><li><p>Let <var title="">timeout</var> be the second argument to
   the method, or zero if the argument was omitted.</p></li>

   <li><p>Apply the ToString() abstract operation to <var title="">timeout</var>, and let <var title="">timeout</var>
   be the result. <a href="references.html#refsECMA262">[ECMA262]</a></p></li>

   <li><p>Apply the ToNumber() abstract operation to <var title="">timeout</var>, and let <var title="">timeout</var> be the
   result. <a href="references.html#refsECMA262">[ECMA262]</a></p></li>

   <li><p>If <var title="">timeout</var> is an Infinity value, a
   Not-a-Number (NaN) value, or negative, let <var title="">timeout</var> be zero.</p></li>

   <li><p>Round <var title="">timeout</var> down to the nearest
   integer, and let <var title="">timeout</var> be the
   result.</p></li>

   <li><p>Return <var title="">timeout</var>.</p></li>

  </ol><hr><p>The <a href="webappapis.html#task-source">task source</a> for these <a href="webappapis.html#concept-task" title="concept-task">tasks</a> is the <dfn id="timer-task-source">timer task
  source</dfn>.</p>

  </div><h3 id="user-prompts"><span class="secno">6.4 </span>User prompts</h3><h4 id="simple-dialogs"><span class="secno">6.4.1 </span>Simple dialogs</h4><dl class="domintro"><dt><var title="">window</var> . <code title="dom-alert"><a href="#dom-alert">alert</a></code>(<var title="">message</var>)</dt>
   <dd>

    <p>Displays a modal alert with the given message, and waits for the user to dismiss it.</p>

    <p>A call to the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method is implied when this method is invoked.</p>

   </dd>

   <dt><var title="">result</var> = <var title="">window</var> . <code title="dom-confirm"><a href="#dom-confirm">confirm</a></code>(<var title="">message</var>)</dt>
   <dd>

    <p>Displays a modal OK/Cancel prompt with the given message, waits
    for the user to dismiss it, and returns true if the user clicks OK
    and false if the user clicks Cancel.</p>

    <p>A call to the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method is implied when this method is invoked.</p>

   </dd>

   <dt><var title="">result</var> = <var title="">window</var> . <code title="dom-prompt"><a href="#dom-prompt">prompt</a></code>(<var title="">message</var> [, <var title="">default</var>] )</dt>
   <dd>

    <p>Displays a modal text field prompt with the given message,
    waits for the user to dismiss it, and returns the value that the
    user entered. If the user cancels the prompt, then returns null
    instead. If the second argument is present, then the given value
    is used as a default.</p>

    <p>A call to the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method is implied when this method is invoked.</p>

   </dd>

  </dl><div class="impl">

  <p>The <dfn id="dom-alert" title="dom-alert"><code>alert(<var title="">message</var>)</code></dfn> method, when invoked, must
  release the <a href="webappapis.html#storage-mutex">storage mutex</a> and show the given <var title="">message</var> to the user. The user agent may make the
  method wait for the user to acknowledge the message before
  returning; if so, the user agent must <a href="webappapis.html#pause">pause</a> while the
  method is waiting.</p>

  <p>The <dfn id="dom-confirm" title="dom-confirm"><code>confirm(<var title="">message</var>)</code></dfn> method, when invoked, must
  release the <a href="webappapis.html#storage-mutex">storage mutex</a> and show the given <var title="">message</var> to the user, and ask the user to respond with
  a positive or negative response. The user agent must then
  <a href="webappapis.html#pause">pause</a> as the method waits for the user's response. If
  the user responds positively, the method must return true, and if
  the user responds negatively, the method must return false.</p>

  <p>The <dfn id="dom-prompt" title="dom-prompt"><code>prompt(<var title="">message</var>, <var title="">default</var>)</code></dfn>
  method, when invoked, must release the <a href="webappapis.html#storage-mutex">storage mutex</a>,
  show the given <var title="">message</var> to the user, and ask the
  user to either respond with a string value or abort. The user agent
  must then <a href="webappapis.html#pause">pause</a> as the method waits for the user's
  response. The second argument is optional. If the second argument
  (<var title="">default</var>) is present, then the response must be
  defaulted to the value given by <var title="">default</var>. If the
  user aborts, then the method must return null; otherwise, the method
  must return the string that the user responded with.</p>

  </div><h4 id="printing"><span class="secno">6.4.2 </span>Printing</h4><dl class="domintro"><dt><var title="">window</var> . <code title="dom-print"><a href="#dom-print">print</a></code>()</dt>

   <dd>

    <p>Prompts the user to print the page.</p>

    <p>A call to the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method is implied when this method is invoked.</p>

   </dd>

  </dl><div class="impl">

  <p>The <dfn id="dom-print" title="dom-print"><code>print()</code></dfn> method,
  when invoked, must run the <a href="#printing-steps">printing steps</a>.</p>

  <p>User agents should also run the <a href="#printing-steps">printing steps</a>
  whenever the user asks for the opportunity to <a href="rendering.html#obtain-a-physical-form">obtain a
  physical form</a> (e.g. printed copy), or the representation of a
  physical form (e.g. PDF copy), of a document.</p>

  <p>The <dfn id="printing-steps">printing steps</dfn> are as follows:</p>

  <ol><li>

    <p>The user agent may display a message to the user and/or may
    abort these steps.</p>

    <p class="example">For instance, a kiosk browser could silently
    ignore any invocations of the <code title="dom-print"><a href="#dom-print">print()</a></code> method.</p>

    <p class="example">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.</p>

   </li>

   <li>

    <p>The user agent must <a href="webappapis.html#fire-a-simple-event">fire a simple event</a> named
    <code title="event-beforeprint">beforeprint</code> at the
    <code><a href="browsers.html#window">Window</a></code> object of the <code><a href="infrastructure.html#document">Document</a></code> that is
    being printed, as well as any <a href="browsers.html#nested-browsing-context" title="nested browsing
    context">nested browsing contexts</a> in it.</p>

    <p class="example">The <code title="event-beforeprint">beforeprint</code> event can be used
    to annotate the printed copy, for instance adding the time at
    which the document was printed.</p>

   </li>

   <li>

    <p>The user agent must release the <a href="webappapis.html#storage-mutex">storage mutex</a>.</p>

   </li>

   <li>

    <p>The user agent should offer the user the opportunity to
    <a href="rendering.html#obtain-a-physical-form">obtain a physical form</a> (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 <a href="webappapis.html#pause">pause</a> 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.</p>

   </li>

   <li>

    <p>The user agent must <a href="webappapis.html#fire-a-simple-event">fire a simple event</a> named
    <code title="event-afterprint">afterprint</code> at the
    <code><a href="browsers.html#window">Window</a></code> object of the <code><a href="infrastructure.html#document">Document</a></code> that is
    being printed, as well as any <a href="browsers.html#nested-browsing-context" title="nested browsing
    context">nested browsing contexts</a> in it.</p>

    <p class="example">The <code title="event-afterprint">afterprint</code> 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.</p>

   </li>

  </ol></div><h4 id="dialogs-implemented-using-separate-documents"><span class="secno">6.4.3 </span>Dialogs implemented using separate documents</h4><dl class="domintro"><dt><var title="">result</var> = <var title="">window</var> . <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog</a></code>(<var title="">url</var> [, <var title="">argument</var>] )</dt>

   <dd>

    <p>Prompts the user with the given page, waits for that page to
    close, and returns the return value.</p>

    <p>A call to the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method is implied when this method is invoked.</p>

   </dd>

  </dl><div class="impl">

  <p>The <dfn id="dom-showmodaldialog" title="dom-showModalDialog"><code>showModalDialog(<var title="">url</var>, <var title="">argument</var>)</code></dfn> method, when invoked, must
  cause the user agent to run the following steps:</p>

  <ol><li>

    <p><a href="urls.html#resolve-a-url" title="resolve a url">Resolve</a> <var title="">url</var> relative to the <a href="browsers.html#entry-script">entry script</a>'s
    <a href="webappapis.html#script-s-base-url" title="script's base URL">base URL</a>.</p>

    <p>If this fails, then throw a <code><a href="common-dom-interfaces.html#syntax_err">SYNTAX_ERR</a></code> exception
    and abort these steps.</p>

   </li>

   <li>

    <p>Release the <a href="webappapis.html#storage-mutex">storage mutex</a>.</p>

   </li>

   <li>

    <p>If the user agent is configured such that this invocation of
    <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> is
    somehow disabled, then return the empty string and abort these
    steps.</p>

    <p class="note">User agents are expected to disable this method in
    certain cases to avoid user annoyance (e.g. as part of their popup
    blocker feature). For instance, a user agent could require that a
    site be white-listed before enabling this method, or the user
    agent could be configured to only allow one modal dialog at a
    time.</p>

   </li>

   <li>

    <p>Let <var title="">the list of background browsing
    contexts</var> be a list of all the browsing contexts that:</p>

    <ul><li>are part of the same <a href="browsers.html#unit-of-related-browsing-contexts">unit of related browsing
     contexts</a> as the browsing context of the
     <code><a href="browsers.html#window">Window</a></code> object on which the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method was
     called, and that</li>

     <li>have an <a href="browsers.html#active-document">active document</a> whose
     <a href="origin-0.html#origin">origin</a> is the <a href="origin-0.html#same-origin" title="same origin">same</a>
     as the <a href="origin-0.html#origin">origin</a> of the <a href="webappapis.html#concept-script" title="concept-script">script</a> that called the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method at
     the time the method was called,</li> 
    </ul><p>...as well as any browsing contexts that are nested inside any
    of the browsing contexts matching those conditions.</p>

   </li>

   <li>

    <p>Disable the user interface for all the browsing contexts in
    <var title="">the list of background browsing contexts</var>. This
    should prevent the user from navigating those browsing contexts,
    causing events to be sent to those browsing context, or editing
    any content in those browsing contexts. However, it does not
    prevent those browsing contexts from receiving events from sources
    other than the user, from running scripts, from running
    animations, and so forth.</p>

   </li>

   <li>

    <p>Create a new <a href="browsers.html#auxiliary-browsing-context">auxiliary browsing context</a>, with the
    <a href="browsers.html#opener-browsing-context">opener browsing context</a> being the browsing context of
    the <code><a href="browsers.html#window">Window</a></code> object on which the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method was
    called. The new auxiliary browsing context has no name.</p>

    <p class="note">This <a href="browsers.html#browsing-context">browsing context</a>'s
    <code><a href="infrastructure.html#document">Document</a></code>s' <code><a href="browsers.html#window">Window</a></code> objects all implement
    the <code><a href="#windowmodal">WindowModal</a></code> interface.</p>

   </li>

   <li>

    <p>Let the <a href="#dialog-arguments">dialog arguments</a> of the new browsing
    context be set to the value of <var title="">argument</var>, or
    the 'undefined' value if the argument was omitted.</p>

   </li>

   <li>

    <p>Let the <a href="#dialog-arguments-origin">dialog arguments' origin</a> be the
    <a href="origin-0.html#origin">origin</a> of the <a href="webappapis.html#concept-script" title="concept-script">script</a> that called the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method.</p>

   </li>

   <li>

    <p><a href="history.html#navigate">Navigate</a> the new
    <a href="browsers.html#browsing-context">browsing context</a> to the <a href="urls.html#absolute-url">absolute URL</a>
    that resulted from <a href="urls.html#resolve-a-url" title="resolve a url">resolving</a>
    <var title="">url</var> earlier, with <a href="history.html#replacement-enabled">replacement
    enabled</a>, and with the <a href="webappapis.html#script-s-browsing-context" title="script's browsing
    context">browsing context</a> of the <a href="webappapis.html#concept-script" title="concept-script">script</a> that invoked the method as
    the <a href="history.html#source-browsing-context">source browsing context</a>.</p>

   </li>

   <li>

    <p><a href="webappapis.html#spin-the-event-loop">Spin the event loop</a> until the new <a href="browsers.html#browsing-context">browsing
    context</a> is closed. (The user agent must allow the user to
    indicate that the <a href="browsers.html#browsing-context">browsing context</a> is to be
    closed.)</p>

   </li>

   <li>

    <p>Reenable the user interface for all the browsing contexts in
    <var title="">the list of background browsing contexts</var>.</p>

   </li>

   <li>

    <p>Return the <a href="browsers.html#auxiliary-browsing-context">auxiliary browsing context</a>'s
    <a href="#return-value">return value</a>.</p>

   </li>

  </ol><p>The <code><a href="browsers.html#window">Window</a></code> objects of <code><a href="infrastructure.html#document">Document</a></code>s hosted
  by <a href="browsers.html#browsing-context" title="browsing context">browsing contexts</a> created
  by the above algorithm must also implement the
  <code><a href="#windowmodal">WindowModal</a></code> interface.</p>

  <p class="note">When this happens, the members of the
  <code><a href="#windowmodal">WindowModal</a></code> interface, in JavaScript environments,
  appear to actually be part of the <code><a href="browsers.html#window">Window</a></code> interface
  (e.g. they are on the same prototype chain as the <code title="dom-alert"><a href="#dom-alert">window.alert()</a></code> method).</p>

  </div><pre class="idl">[NoInterfaceObject] interface <dfn id="windowmodal">WindowModal</dfn> {
  readonly attribute any <a href="#dom-windowmodal-dialogarguments" title="dom-WindowModal-dialogArguments">dialogArguments</a>;
           attribute DOMString <a href="#dom-windowmodal-returnvalue" title="dom-WindowModal-returnValue">returnValue</a>;
};</pre><dl class="domintro"><dt><var title="">window</var> . <code title="dom-WindowModal-dialogArguments"><a href="#dom-windowmodal-dialogarguments">dialogArguments</a></code></dt>

   <dd>

    <p>Returns the <var title="">argument</var> argument that was
    passed to the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method.</p>

   </dd>

   <dt><var title="">window</var> . <code title="dom-WindowModal-returnValue"><a href="#dom-windowmodal-returnvalue">returnValue</a></code> [ = <var title="">value</var> ]</dt>

   <dd>

    <p>Returns the current return value for the window.</p>

    <p>Can be set, to change the value that will be returned by the
    <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code>
    method.</p>

   </dd>

  </dl><div class="impl">

  <p>Such browsing contexts have associated <dfn id="dialog-arguments">dialog
  arguments</dfn>, which are stored along with the <dfn id="dialog-arguments-origin">dialog
  arguments' origin</dfn>. These values are set by the <code title="dom-showModalDialog"><a href="#dom-showmodaldialog">showModalDialog()</a></code> method in the
  algorithm above, when the browsing context is created, based on the
  arguments provided to the method.</p>

  <p>The <dfn id="dom-windowmodal-dialogarguments" title="dom-WindowModal-dialogArguments"><code>dialogArguments</code></dfn>
  IDL attribute, on getting, must check whether its browsing context's
  <a href="browsers.html#active-document">active document</a>'s <a href="origin-0.html#origin">origin</a> is the <a href="origin-0.html#same-origin" title="same origin">same</a> as the <a href="#dialog-arguments-origin">dialog arguments'
  origin</a>. If it is, then the browsing context's <a href="#dialog-arguments">dialog
  arguments</a> must be returned unchanged. Otherwise, if the
  <a href="#dialog-arguments">dialog arguments</a> are an object, then the empty string
  must be returned, and if the <a href="#dialog-arguments">dialog arguments</a> are not
  an object, then the stringification of the <a href="#dialog-arguments">dialog
  arguments</a> must be returned.

  </p><p>These browsing contexts also have an associated <dfn id="return-value">return
  value</dfn>. The <a href="#return-value">return value</a> of a browsing context
  must be initialized to the empty string when the browsing context is
  created.</p>

  <p>The <dfn id="dom-windowmodal-returnvalue" title="dom-WindowModal-returnValue"><code>returnValue</code></dfn>
  IDL attribute, on getting, must return the <a href="#return-value">return value</a>
  of its browsing context, and on setting, must set the <a href="#return-value">return
  value</a> to the given new value.</p>

  </div><p class="note">The <code title="dom-window-close"><a href="browsers.html#dom-window-close">window.close()</a></code> method can be used to
  close the browsing context.</p><h3 id="system-state-and-capabilities:-the-navigator-object"><span class="secno">6.5 </span>System state and capabilities: the <code><a href="#navigator">Navigator</a></code> object</h3><div class="impl">

  <p>The <dfn id="dom-navigator" title="dom-navigator"><code>navigator</code></dfn>
  attribute of the <code><a href="browsers.html#window">Window</a></code> interface must return an
  instance of the <code><a href="#navigator">Navigator</a></code> 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:</p>

  </div><pre class="idl">interface <dfn id="navigator">Navigator</dfn> {
  // objects implementing this interface also implement the interfaces given below
};
<a href="#navigator">Navigator</a> implements <a href="#navigatorid">NavigatorID</a>;
<a href="#navigator">Navigator</a> implements <a href="offline.html#navigatoronline">NavigatorOnLine</a>;
<a href="#navigator">Navigator</a> implements <a href="#navigatorcontentutils">NavigatorContentUtils</a>;
<a href="#navigator">Navigator</a> implements <a href="#navigatorstorageutils">NavigatorStorageUtils</a>;</pre><div class="impl">

  <p>These interfaces are defined separately so that other
  specifications can re-use parts of the <code><a href="#navigator">Navigator</a></code>
  interface.</p>

   

   
  </div><h4 id="client-identification"><span class="secno">6.5.1 </span>Client identification</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="navigatorid">NavigatorID</dfn> {  readonly attribute DOMString <a href="#dom-navigator-appname" title="dom-navigator-appName">appName</a>;
  readonly attribute DOMString <a href="#dom-navigator-appversion" title="dom-navigator-appVersion">appVersion</a>;
  readonly attribute DOMString <a href="#dom-navigator-platform" title="dom-navigator-platform">platform</a>;
  readonly attribute DOMString <a href="#dom-navigator-useragent" title="dom-navigator-userAgent">userAgent</a>;
};</pre><p>In certain cases, despite the best efforts of the entire
  industry, Web browsers have bugs and limitations that Web authors
  are forced to work around.</p><p>This section defines a collection of attributes that can be used
  to determine, from script, the kind of user agent in use, in order
  to work around these issues.</p><p>Client detection should always be limited to detecting known
  current versions; future versions and unknown versions should always
  be assumed to be fully compliant.</p><dl class="domintro"><dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-appName"><a href="#dom-navigator-appname">appName</a></code></dt>
   <dd>
    <p>Returns the name of the browser.</p>
   </dd>

   <dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-appVersion"><a href="#dom-navigator-appversion">appVersion</a></code></dt>
   <dd>
    <p>Returns the version of the browser.</p>
   </dd>

   <dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-platform"><a href="#dom-navigator-platform">platform</a></code></dt>
   <dd>
    <p>Returns the name of the platform.</p>
   </dd>

   <dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-userAgent"><a href="#dom-navigator-useragent">userAgent</a></code></dt>
   <dd>
    <p>Returns the complete User-Agent header.</p>
   </dd>

  </dl><div class="impl">

  <dl><dt><dfn id="dom-navigator-appname" title="dom-navigator-appName"><code>appName</code></dfn></dt>
   <dd><p>Must return either the string "<code title="">Netscape</code>" or the full name of the browser, e.g. "<code title="">Mellblom Browsernator</code>".</p></dd>

   <dt><dfn id="dom-navigator-appversion" title="dom-navigator-appVersion"><code>appVersion</code></dfn></dt>
   <dd><p>Must return either the string "<code title="">4.0</code>" or a string representing the version of the browser in detail, e.g. "<code title="">1.0 (VMS; en-US) Mellblomenator/9000</code>".</p></dd>

   

   

   <dt><dfn id="dom-navigator-platform" title="dom-navigator-platform"><code>platform</code></dfn></dt>
   <dd><p>Must return either the empty string or a string representing the platform on which the browser is executing, e.g. "<code title="">MacIntel</code>", "<code title="">Win32</code>", "<code title="">FreeBSD i386</code>", "<code title="">WebTV OS</code>".</p></dd>

   

   

   

   

   <dt><dfn id="dom-navigator-useragent" title="dom-navigator-userAgent"><code>userAgent</code></dfn></dt>
   <dd><p>Must return the string used for the value of the "<code title="">User-Agent</code>" header in HTTP requests, or the empty string if no such header is ever sent.</p></dd>

   

   

  </dl><p class="warning">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.</p>

  </div><h4 id="custom-handlers"><span class="secno">6.5.2 </span>Custom scheme and content handlers</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="navigatorcontentutils">NavigatorContentUtils</dfn> {
  // content handler registration
  void <a href="#dom-navigator-registerprotocolhandler" title="dom-navigator-registerProtocolHandler">registerProtocolHandler</a>(in DOMString scheme, in DOMString url, in DOMString title);
  void <a href="#dom-navigator-registercontenthandler" title="dom-navigator-registerContentHandler">registerContentHandler</a>(in DOMString mimeType, in DOMString url, in DOMString title);
};</pre><p>The <dfn id="dom-navigator-registerprotocolhandler" title="dom-navigator-registerProtocolHandler"><code>registerProtocolHandler()</code></dfn>
  method allows Web sites to register themselves as possible handlers
  for particular schemes. For example, an online telephone messaging
  service could register itself as a handler of the <code>sms:</code>
  scheme (<a href="references.html#refsRFC5724">[RFC5724]</a>), so that if the user
  clicks on such a link, he is given the opportunity to use that Web
  site. Analogously, the <dfn id="dom-navigator-registercontenthandler" title="dom-navigator-registerContentHandler"><code>registerContentHandler()</code></dfn>
  method allows Web sites to register themselves as possible handlers
  for content in a particular <a href="infrastructure.html#mime-type">MIME type</a>. For example, the
  same online telephone messaging service could register itself as a
  handler for <code>text/directory</code> files (<a href="references.html#refsRFC2425">[RFC2425]</a>), so that if the user has no
  native application capable of handling vCards (<a href="references.html#refsRFC2426">[RFC2426]</a>), his Web browser can instead
  suggest he use that site to view contact information stored on
  vCards that he opens.</p><dl class="domintro"><dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-registerProtocolHandler"><a href="#dom-navigator-registerprotocolhandler">registerProtocolHandler</a></code>(<var title="">scheme</var>, <var title="">url</var>, <var title="">title</var>)</dt>
   <dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-registerContentHandler"><a href="#dom-navigator-registercontenthandler">registerContentHandler</a></code>(<var title="">mimeType</var>, <var title="">url</var>, <var title="">title</var>)</dt>

   <dd>

    <p>Registers a handler for the given scheme or content type, at
    the given URL, with the given title.</p>

    <p>The string "<code title="">%s</code>" in the URL is used as a
    placeholder for where to put the URL of the content to be
    handled.</p>

    <p>Throws a <code><a href="common-dom-interfaces.html#security_err">SECURITY_ERR</a></code> exception if the user agent
    blocks the registration (this might happen if trying to register
    as a handler for "http", for instance).</p>

    <p>Throws a <code><a href="common-dom-interfaces.html#syntax_err">SYNTAX_ERR</a></code> if the "<code title="">%s</code>" string is missing in the URL.</p>

   </dd>

  </dl><div class="impl">

  <p>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.</p>

  <p>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.</p>

  <p>The arguments to the methods have the following meanings and
  corresponding implementation requirements:</p>

  <dl><dt><var title="">scheme</var> (<code title="dom-navigator-registerProtocolHandler"><a href="#dom-navigator-registerprotocolhandler">registerProtocolHandler()</a></code> only)</dt>

   <dd>

    <p>A scheme, such as <code>ftp</code> or <code>sms</code>. The
    scheme must be compared in an <a href="infrastructure.html#ascii-case-insensitive">ASCII case-insensitive</a>
    manner by user agents for the purposes of comparing with the
    scheme part of URLs that they consider against the list of
    registered handlers.</p>

    <p>The <var title="">scheme</var> value, if it contains a colon
    (as in "<code>ftp:</code>"), will never match anything, since
    schemes don't contain colons.</p>

    <p class="note">This feature is not intended to be used with
    non-standard protocols.</p>

   </dd>

   <dt><var title="">mimeType</var> (<code title="dom-navigator-registerContentHandler"><a href="#dom-navigator-registercontenthandler">registerContentHandler()</a></code> only)</dt>

   <dd>

    <p>A <a href="infrastructure.html#mime-type">MIME type</a>, such as
    <code>model/vnd.flatland.3dml</code> or
    <code>application/vnd.google-earth.kml+xml</code>. The <a href="infrastructure.html#mime-type">MIME
    type</a> must be compared in an <a href="infrastructure.html#ascii-case-insensitive">ASCII
    case-insensitive</a> manner by user agents for the purposes of
    comparing with MIME types of documents that they consider against
    the list of registered handlers.</p>

    <p>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 <var title="">mimeType</var> 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.</p>

    <p class="note">The type is compared to the <a href="infrastructure.html#mime-type">MIME type</a>
    used by the user agent <em>after</em> the sniffing algorithms have
    been applied.</p>

   </dd>


   <dt><var title="">url</var></dt>

   <dd>

    <p>A string used to build the <a href="urls.html#url">URL</a> of the page that
    will handle the requests.</p>

    <p>When the user agent uses this URL, it must replace the first
    occurrence of the exact literal string "<code title="">%s</code>"
    with an escaped version of the <a href="urls.html#absolute-url">absolute URL</a> of the
    content in question (as defined below), then <a href="urls.html#resolve-a-url" title="resolve
    a url">resolve</a> the resulting URL, relative to the <a href="webappapis.html#script-s-base-url" title="script's base URL">base URL</a> of the <a href="browsers.html#entry-script">entry
    script</a> at the time the <code title="dom-navigator-registerContentHandler"><a href="#dom-navigator-registercontenthandler">registerContentHandler()</a></code>
    or <code title="dom-navigator-registerProtocolHandler"><a href="#dom-navigator-registerprotocolhandler">registerProtocolHandler()</a></code>
    methods were invoked, and then <a href="history.html#navigate">navigate</a> an appropriate <a href="browsers.html#browsing-context">browsing context</a> to the
    resulting URL using the GET method (<a href="fetching-resources.html#concept-http-equivalent-get" title="concept-http-equivalent-get">or equivalent</a> for
    non-HTTP URLs).</p>

    <p>To get the escaped version of the <a href="urls.html#absolute-url">absolute URL</a> of
    the content in question, the user agent must replace every
    character in that <a href="urls.html#absolute-url">absolute URL</a> that doesn't match the
    &lt;query&gt; production defined in RFC 3986 by the
    percent-encoded form of that character. <a href="references.html#refsRFC3986">[RFC3986]</a></p>

    <div class="example">

     <p>If the user had visited a site at <code title="">http://example.com/</code> that made the following
     call:</p>

     <pre>navigator.registerContentHandler('application/x-soup', 'soup?url=%s', 'SoupWeb&#8482;')</pre>

     <p>...and then, much later, while visiting <code title="">http://www.example.net/</code>, clicked on a link such
     as:</p>

     <pre>&lt;a href="chickenk&#239;wi.soup"&gt;Download our Chicken K&#239;wi soup!&lt;/a&gt;</pre>

     <p>...then, assuming this <code>chickenk&#239;wi.soup</code> file
     was served with the <a href="infrastructure.html#mime-type">MIME type</a>
     <code>application/x-soup</code>, the UA might navigate to the
     following URL:</p>

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

     <p>This site could then fetch the <code>chickenk&#239;wi.soup</code>
     file and do whatever it is that it does with soup (synthesize it
     and ship it to the user, or whatever).</p>

    </div>

   </dd>

   <dt><var title="">title</var></dt>

   <dd>

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

   </dd>

  </dl><p>User agents should raise <code><a href="common-dom-interfaces.html#security_err">SECURITY_ERR</a></code> exceptions if
  the methods are called with <var title="">scheme</var> or <var title="">mimeType</var> values that the UA deems to be
  "privileged". For example, a site attempting to register a handler
  for <code>http</code> URLs or <code><a href="iana.html#text-html">text/html</a></code> content in a
  Web browser would likely cause an exception to be raised.</p>

  <p>User agents must raise a <code><a href="common-dom-interfaces.html#syntax_err">SYNTAX_ERR</a></code> exception if the
  <var title="">url</var> argument passed to one of these methods does
  not contain the exact literal string "<code>%s</code>", or if <a href="urls.html#resolve-a-url" title="resolve a url">resolving</a> the <var title="">url</var>
  argument with the first occurrence of the string "<code title="">%s</code>" removed, relative to the <a href="browsers.html#entry-script">entry
  script</a>'s <a href="webappapis.html#script-s-base-url" title="script's base URL">base URL</a>, is
  not successful.</p>

  <p>User agents must not raise any other exceptions (other than
  binding-specific exceptions, such as for an incorrect number of
  arguments in an JavaScript implementation).</p>

  <p>This section does not define how the pages registered by these
  methods are used, beyond the requirements on how to process the <var title="">url</var> value (see above). To some extent, the <a href="history.html#navigate" title="navigate">processing model for navigating across
  documents</a> 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.</p>

  <p>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.</p>

  </div><div class="impl">

  <h5 id="security-and-privacy"><span class="secno">6.5.2.1 </span>Security and privacy</h5>

  <p>These mechanisms can introduce a number of concerns, in
  particular privacy concerns.</p>

  <p><strong>Hijacking all Web usage.</strong> User agents should not
  allow schemes that are key to its normal operation, such as
  <code>http</code> or <code>https</code>, 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.</p>

  <p><strong>Hijacking defaults.</strong> It is strongly recommended
  that user agents do 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.</p>

  <p><strong>Registration spamming.</strong> 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 <code>video/mpeg</code> &#8212;
  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.</p>

  <p><strong>Misleading titles.</strong> User agents should not rely
  wholly on the <var title="">title</var> argument to the methods when
  presenting the registered handlers to the user, since sites could
  easily lie. For example, a site <code>hostile.example.net</code>
  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.</p>

  <p><strong>Hostile handler metadata.</strong> 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.</p>

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

  <ol><li>The user registers a third-party content handler as the default
   handler for a content type.</li>

   <li>The user then browses his corporate Intranet site and accesses
   a document that uses that content type.</li>

   <li>The user agent contacts the third party and hands the third
   party the URL to the Intranet content.</li>

  </ol><p>No actual confidential file data is leaked in this manner, but
  the URLs themselves could contain confidential information. For
  example, the URL could be
  <code>http://www.corp.example.com/upcoming-aquisitions/the-sample-company.egf</code>,
  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.</p>

  <p><strong>Leaking secure URLs.</strong> User agents should not send
  HTTPS URLs to third-party sites registered as content handlers, in
  the same way that user agents do not send <code title="http-referer">Referer</code> (sic) HTTP headers from secure
  sites to third-party sites.</p>

  <p><strong>Leaking credentials.</strong> 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).</p>

  </div><div class="impl">

  <h5 id="sample-handler-impl"><span class="secno">6.5.2.2 </span>Sample user interface</h5>

  <p><i>This section is non-normative.</i></p>

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

  <p>The <code title="dom-navigator-registerContentHandler"><a href="#dom-navigator-registercontenthandler">registerContentHandler()</a></code>
  method could display a modal dialog box:</p>

  <p><img alt="The modal dialog box could have the title 'Content Handler Registration', and could say 'This Web page: Kittens at work http://kittens.example.org/ ...would like permission to handle files of type: application/x-meowmeow using the following Web-based application: Kittens-at-work displayer http://kittens.example.org/?show=%s Do you trust the administrators of the &quot;kittens.example.org&quot; domain?' with two buttons, 'Trust kittens.example.org' and 'Cancel'." height="374" src="sample-content-handler-registration.png" width="534"></p>

  <p>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 <code title="dom-navigator-registerContentHandler"><a href="#dom-navigator-registercontenthandler">registerContentHandler()</a></code>
  method as its first argument (<var title="">mimeType</var>),
  "http://kittens.example.org/?show=%s" was the second argument (<var title="">url</var>), and "Kittens-at-work displayer" was the third
  argument (<var title="">title</var>).</p>

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

  <p>When the user then attempts to fetch a URL that uses the
  "application/x-meowmeow" <a href="infrastructure.html#mime-type">MIME type</a>, then it might
  display a dialog as follows:</p>

  <p><img alt="The dialog box could have the title 'Unknown File Type' and could say 'You have attempted to access:' followed by a URL, followed by a prompt such as 'How would you like FerretBrowser to handle this resource?' with three radio buttons, one saying 'Contact the FerretBrowser plugin registry to see if there is an official way to handle this resource.', one saying 'Pass this URL to a local application' with an application selector, and one saying 'Pass this URL to the &quot;Kittens-at-work displayer&quot; application at &quot;kittens.example.org&quot;', with a checkbox labeled 'Always do this for resources using the &quot;application/x-meowmeow&quot; type in future.', and with two buttons, 'Ok' and 'Cancel'." height="428" src="sample-content-handler.png" width="577"></p>

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

  <p>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".</p>

  <p>The <code title="dom-navigator-registerProtocolHandler"><a href="#dom-navigator-registerprotocolhandler">registerProtocolHandler()</a></code>
  method would work equivalently, but for schemes instead of unknown
  content types.</p>

  </div><h4 id="manually-releasing-the-storage-mutex"><span class="secno">6.5.3 </span>Manually releasing the storage mutex</h4><pre class="idl">[Supplemental, NoInterfaceObject]
interface <dfn id="navigatorstorageutils">NavigatorStorageUtils</dfn> {
  void <a href="#dom-navigator-yieldforstorageupdates" title="dom-navigator-yieldForStorageUpdates">yieldForStorageUpdates</a>();
};</pre><dl class="domintro"><dt><var title="">window</var> . <code title="dom-navigator"><a href="#dom-navigator">navigator</a></code> . <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">yieldForStorageUpdates</a></code>()</dt>

   <dd>

    <p>If a script uses the <code title="dom-document-cookie"><a href="dom.html#dom-document-cookie">document.cookie</a></code> API, or the
    <code title="dom-localStorage">localStorage</code> API, the
    browser will block other scripts from accessing cookies or storage
    until the first script finishes.
    
    <a href="references.html#refsWEBSTORAGE">[WEBSTORAGE]</a>
    
    </p>

    <p>Calling the <code title="dom-navigator-yieldForStorageUpdates"><a href="#dom-navigator-yieldforstorageupdates">navigator.yieldForStorageUpdates()</a></code>
    method tells the user agent to unblock any other scripts that may
    be blocked, even though the script hasn't returned.</p>

    <p>Values of cookies and items in the <code>Storage</code> objects
    of <code title="dom-localStorage">localStorage</code> attributes
    can change after calling this method, whence its name.
    
    <a href="references.html#refsWEBSTORAGE">[WEBSTORAGE]</a>
    
    </p>

   </dd>

  </dl><div class="impl">

   <p>The <dfn id="dom-navigator-yieldforstorageupdates" title="dom-navigator-yieldForStorageUpdates"><code>yieldForStorageUpdates()</code></dfn>
   method, when invoked, must, if the <a href="webappapis.html#storage-mutex">storage mutex</a> is
   owned by the <a href="webappapis.html#event-loop">event loop</a> of the <a href="webappapis.html#concept-task" title="concept-task">task</a> that resulted in the method being
   called, release the <a href="webappapis.html#storage-mutex">storage mutex</a> so that it is once
   again free. Otherwise, it must do nothing.</p>

  </div></body></html>