Specify wheel event groups

Specify the behavior of wheel event groups.

sections/event-types.txt

@@ -2068,6 +2068,18 @@ myDiv.addEventListener("auxclick", function(e) {
 	deltaY value respectively).
 	</p>
 
+	A <a>user agent</a> MUST create a <a>wheel event transaction</a> when the first wheel event
+	is fired, so that all subsequent wheel events within a implementation-specific amount of
+	time can be targetted at the same element. A <dfn>wheel event transaction</dfn> is series of
+	wheel events that are associated with a single user gesture. The
+	<a>wheel event transaction</a> MUST have an associated event target that is the
+	<a>topmost event target</a> at the time the first wheel event occurs in the group.
+
+	<p class="example">
+	If a series of wheel events targetted in a scrollable element start above a child element,
+	later events for the same user gesture may occur over the child element.
+	</p>
+
 	<h4 id="interface-wheelevent">Interface WheelEvent</h4>
 
 		<p class="intro-dom">Introduced in this specification</p>
@@ -2229,7 +2241,8 @@ myDiv.addEventListener("auxclick", function(e) {
 			+| Composed         | Yes                                                                                  |
 			+| Default action   | Scroll (or zoom) the document                                                        |
 			+| Context<br/>     | <ul>                                                                                 |
-			 | (trusted events) | <li>{{Event}}.{{Event/target}} : <a>topmost event target</a></li>                    |
+			 | (trusted events) | <li>{{Event}}.{{Event/target}} : element target for the current                      |
+			 |                  |     <a>wheel event transaction</a>                                                   |
 			 |                  | <li>{{UIEvent}}.{{UIEvent/view}} : <a><code>Window</code></a></li>                   |
 			 |                  | <li>{{UIEvent}}.{{UIEvent/detail}} : <code>0</code></li>                             |
 			 |                  | <li>{{MouseEvent}}.{{MouseEvent/screenX}} : if the wheel is associated with a        |