Fix event handler processing algorithm special cases

This is a follow-up to 7163372, which
forgot about the case of ErrorEvents with a non-"error" event type, or
BeforeUnloadEvents with a non-"beforeunload" event type, as noted in
whatwg#2398 (comment).

source

@@ -88691,18 +88691,12 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
    <li><p>If <var>callback</var> is null, then abort these steps.</p></li>
 
-   <li>
-    <p>Let <var>special error event handling</var> be true if <var>E</var> is an
-    <code>ErrorEvent</code> object and <var>E</var>'s <code
-    data-x="dom-Event-currentTarget">currentTarget</code> implements the
-    <code>WindowOrWorkerGlobalScope</code> mixin. Otherwise, let <var>special error event
-    handling</var> be false.</p>
-
-    <p class="note">In this case, <var>E</var>'s <code data-x="dom-Event-type">type</code> will
-    always be <code data-x="event-error">error</code>, since the only <span data-x="event
-    handlers">event handler</span> is called with <code>ErrorEvent</code> instances is <code
-    data-x="handler-onerror">onerror</code>.</p>
-   </li>
+   <li><p>Let <var>special error event handling</var> be true if <var>E</var> is an
+   <code>ErrorEvent</code> object, <var>E</var>'s <code data-x="dom-Event-type">type</code> is <code
+   data-x="event-error">error</code>, and <var>E</var>'s <code
+   data-x="dom-Event-currentTarget">currentTarget</code> implements the
+   <code>WindowOrWorkerGlobalScope</code> mixin. Otherwise, let <var>special error event
+   handling</var> be false.</p></li>
 
    <li>
 
@@ -88752,15 +88746,17 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
     <dl class="switch">
 
-     <dt>If <var>E</var> is a <code>BeforeUnloadEvent</code> object</dt>
+     <dt>If <var>E</var> is a <code>BeforeUnloadEvent</code> object and <var>E</var>'s <code
+     data-x="dom-Event-type">type</code> is <code
+     data-x="event-beforeunload">beforeunload</code></dt>
+     <!-- It is possible to get a BeforeUnloadEvent object whose type is not beforeunload via
+     document.createEvent("beforeunloadevent") + initEvent. This may be removed; see
+     https://github.com/whatwg/dom/issues/362. But it's still probably a good idea to check both,
+     just in case e.g. we end up adding a constructor to BeforeUnloadEvent. -->
      <dd>
-      <p class="note">This can only occur when <var>E</var>'s <code
-      data-x="dom-Event-type">type</code> is <code data-x="event-beforeunload">beforeunload</code>
-      and when the <span data-x="event handler IDL attributes">event handler IDL attribute</span>'s
-      type is <code>OnBeforeUnloadEventHandler</code>, since the only <span data-x="event
-      handlers">event handler</span> that is called with <code>BeforeUnloadEvent</code> instances is
-      <code data-x="handler-window-onbeforeunload">onbeforeunload</code>. In this case, <var>return
-      value</var> will therefore have been coerced into either the value null or a <code
+      <p class="note">In this case, the <span data-x="event handler IDL attributes">event handler
+      IDL attribute</span>'s type will be <code>OnBeforeUnloadEventHandler</code>, so <var>return
+      value</var> will have been coerced into either null or a <code
       data-x="idl-DOMString">DOMString</code>.</p>
 
       <p>If <var>return value</var> is not null, then:</p>
@@ -88780,8 +88776,17 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
      flag</span>.</p></dd>
 
      <dt>Otherwise</dt>
-     <dd><p>If <var>return value</var> is false, then set <var>E</var>'s <span>canceled
-     flag</span>.</p></dd>
+     <dd>
+      <p>If <var>return value</var> is false, then set <var>E</var>'s <span>canceled
+      flag</span>.</p>
+
+      <p class="note">If we've gotten to this "Otherwise" clause because <var>E</var>'s <code
+      data-x="dom-Event-type">type</code> is <code
+      data-x="event-beforeunload">beforeunload</code> but <var>E</var> is <em>not</em> a
+      <code>BeforeUnloadEvent</code> object, then <var>return value</var> will never be false, since
+      in such cases <var>return value</var> will have been coerced into either null or a <code
+      data-x="idl-DOMString">DOMString</code>.</p>
+     </dd>
 
     </dl>