[css-transitions-2] Add transitionrun event and define transitionstart to fire after delay

These definitions still need some work with regards to defining the value of
elapsedTime but that is true for level 1 of CSS Animations too (see issue w3c#68).

This closes w3c#416.

Author: birtles

css-transitions-2/Overview.bs

@@ -130,9 +130,19 @@ to the <a>global animation list</a> at the moment they are constructed.
 	<p>The <em>additional</em> types of transition events that can occur are:</p>
 
 	<dl dfn-type=event dfn-for=transitionevent>
+		<dt><dfn>transitionrun</dfn>
+		<dd>
+			The <a idl>transitionrun</a> event occurs when a transition is created (i.e., when it is added to the set of <a>running transitions</a>).
+			<ul>
+				<li>Bubbles: Yes</li>
+				<li>Cancelable: No</li>
+				<li>Context Info: propertyName, elapsedTime, pseudoElement</li>
+			</ul>
 		<dt><dfn>transitionstart</dfn>
 		<dd>
-			The <a idl>transitionstart</a> event occurs when a transition is started (i.e., when it is added to the set of <a>running transitions</a>).
+			The <a idl>transitionstart</a> event occurs when a transition's delay
+			phase ends. If the transition has a negative 'transition-delay', the
+			event will have an elapsedTime equal to the absolute value of the delay.
 			<ul>
 				<li>Bubbles: Yes</li>
 				<li>Cancelable: No</li>
@@ -148,6 +158,8 @@ to the <a>global animation list</a> at the moment they are constructed.
 			</ul>
 	</dl>
 
+Issue: Define precisely the value of elapsedTime for each of these events.
+
 # DOM Interfaces # {#interface-dom}
 
 ## The CSSTransition interface ## {#the-CSSTransition-interface}