Update intrinsic iteration duration for a group effect

Author: Kevin Ellis

web-animations-2/Overview.bs

@@ -1555,7 +1555,11 @@ number of <a>child effects</a> as follows.
 
     1.  Let <var>maximum end time</var> be the maximum value
         after calculating the <a>end time</a> of each <a>child
-        effect</a> in the group.
+        effect</a> in the group. The <a>end time</a> may be
+        expressed as a time or a percentage (in the case of a
+        progress based animation). In the event of mixed time and
+        percentage <a>end time</a>s, the largest time based value
+        equates to 100% for the purpose of time scaling.
     2.  The <a>intrinsic iteration duration</a> is the result of
         evaluating <code>max(0, <var>maximum end
         time</var>)</code>.