Ignore beforeunload event return value

Instead of using it as the message to show to the user, simply compare
it against the empty string or not. This matches Gecko, WebKit, and
Blink (but not yet EdgeHTML).

Closes #952.

source

@@ -3104,7 +3104,7 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
      <li><dfn data-noexport="" data-x="dom-Node-appendChild" data-x-href="https://dom.spec.whatwg.org/#dom-node-appendchild"><code>appendChild()</code></dfn> method</li>
      <li><dfn data-noexport="" data-x="dom-Node-cloneNode" data-x-href="https://dom.spec.whatwg.org/#dom-node-clonenode"><code>cloneNode()</code></dfn> method</li>
      <li><dfn data-noexport="" data-x="dom-Document-importNode" data-x-href="https://dom.spec.whatwg.org/#dom-document-importnode"><code>importNode()</code></dfn> method</li>
-
+     <li><dfn data-noexport="" data-x="dom-Event-preventDefault" data-x-href="https://dom.spec.whatwg.org/#dom-event-preventdefault"><code>preventDefault()</code></dfn> method</li>
      <li><dfn data-noexport="" data-x="dom-Element-id" data-x-href="https://dom.spec.whatwg.org/#dom-element-id"><code>id</code></dfn> attribute</li>
      <li><dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#dom-node-textcontent"><code>textContent</code></dfn> attribute</li>
 
@@ -35734,10 +35734,11 @@ function playSound(id) {
 sfx.oncanplaythrough = function () {
   playSound('dog bark');
 }
-// meow when the user tries to leave
-window.onbeforeunload = function () {
+// meow when the user tries to leave,
+// and have the browser ask them to stay
+window.onbeforeunload = function (e) {
   playSound('kitten mew');
-  return 'Are you sure you want to leave this awesome page?';
+  e.preventDefault();
 }</pre>
 
   </div>
@@ -82214,9 +82215,9 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
     object is not the empty string, or if the event was canceled, then the user agent should ask the
     user to confirm that they wish to unload the document.</p>
 
-    <p>The prompt shown by the user agent may include the string of the <code
-    data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute, <span
-    data-x="optionally truncate a simple dialog string">optionally truncated</span>.</p>
+    <p class="note">The message shown to the user is not customizable, but instead determined by
+    the user agent. In particular, the actual value of the <code
+    data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute is ignored.</p>
 
     <p>The user agent must <span>pause</span> while waiting for the user's response.</p>
 
@@ -82391,29 +82392,28 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
   attribute DOMString <span data-x="dom-BeforeUnloadEvent-returnValue">returnValue</span>;
 };</pre>
 
-  <dl class="domintro">
-
-   <dt><var>event</var> . <code subdfn data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> [ = <var>value</var> ]</dt>
-
-   <dd>
-
-    <p>Returns the current return value of the event (the message to show the user).</p>
-
-    <p>Can be set, to update the message.</p>
-
-   </dd>
-
-  </dl>
-
   <p class="note">There are no <code>BeforeUnloadEvent</code>-specific initialisation methods.</p>
 
+  <p>The <code>BeforeUnloadEvent</code> interface is a legacy interface which allows <span
+  data-x="prompt to unload a document">prompting to unload the document</span> to be controlled
+  not only by canceling the event, but by setting the <code
+  data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute to a value besides the
+  empty string. Authors should use the <code
+  data-x="dom-Event-preventDefault">preventDefault()</code> method, or other means of canceling
+  events, instead of using <code data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code>.</p>
+
   <div w-nodev>
 
   <p>The <dfn><code data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code></dfn> attribute
-  represents the message to show the user. When the event is created, the attribute must be set to
+  controls the process of <span data-x="prompt to unload a document">prompting to unload the
+  document</span>. When the event is created, the attribute must be set to
   the empty string. On getting, it must return the last value it was set to. On setting, the
   attribute must be set to the new value.</p>
 
+  <p class="note">This attribute is a <code data-x="">DOMString</code> only for historical reasons.
+  Any value besides the empty string will be treated as a request to ask the user for
+  confirmation.</p>
+
   </div>
 
 
@@ -87031,7 +87031,7 @@ typedef <span>EventHandlerNonNull</span>? <dfn>EventHandler</dfn>;</pre>
   <span w-nodev>as described above,</span> if the return value is false, the event is canceled
   (except for <code data-x="event-mouseover">mouseover</code> events, where the return value has to
   be true to cancel the event). With <code data-x="event-beforeunload">beforeunload</code> events,
-  the value is instead used to determine the message to show the user.</p>
+  the value is instead used to determine whether or not to prompt about unloading the document.</p>
 
   <p>For historical reasons, the <code data-x="handler-onerror">onerror</code> handler has different
   arguments:</p>