Specify wheel event groups

Specify the behavior of wheel event groups.

sections/event-types.txt

@@ -2180,11 +2180,12 @@ myDiv.addEventListener("auxclick", function(e) {
 			+| Sync / Async     | Async                                                                                |
 			+| Bubbles          | Yes                                                                                  |
 			+| Trusted Targets  | <code>Element</code>                                                                 |
-			+| Cancelable       | <a href="#cancelability-of-wheel-events">Varies</a>                                                  |
+			+| Cancelable       | <a href="#cancelability-of-wheel-events">Varies</a>                                  |
 			+| 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 wheel event group or |
+			 |                  |     if no wheel event group has started, then the <a>topmost event target</a></li>   |
 			 |                  | <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        |
@@ -2236,6 +2237,19 @@ myDiv.addEventListener("auxclick", function(e) {
 			EVENT{wheel} event with multiple non-zero axes or as separate
 			EVENT{wheel} events for each non-zero axis.
 
+			A <a>user agent</a> SHOULD create a wheel event group 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.
+
+			<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. In the case that {{Event/preventDefault()}}
+			is called in a wheel event handler for the child element, targetting
+			the child element could result in unexpected behavior for the user.
+			</p>
+
 			The typical <a>default action</a> of the EVENT{wheel} event type is
 			to scroll (or in some cases, zoom) the document by the indicated
 			amount.  If this event is canceled, the implementation MUST NOT