[css-animations-2] Add delta specification with an animationcancel event.

This is per WG resolution at
https://lists.w3.org/Archives/Public/www-style/2015Apr/0405.html .

Author: dbaron

css-animations-2/Overview.bs

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+<h1>CSS Animations Level 2</h1>
+
+<pre class='metadata'>
+Status: ED
+Shortname: css-animations-2
+Level: 2
+Group: csswg
+ED: http://dev.w3.org/csswg/css-animations-2/
+Editor: L. David Baron, Mozilla, dbaron@dbaron.org
+!Issues List: <a href="https://www.w3.org/Bugs/Public/buglist.cgi?component=Animations&list_id=36653&product=CSS&query_format=advanced&resolution=---">In Bugzilla</a>
+
+Abstract: This CSS module describes a way for authors to animate the values of CSS properties over time, using keyframes. The behavior of these keyframe animations can be controlled by specifying their duration, number of repeats, and repeating behavior.
+</pre>
+
+<h2 id="delta">Delta specification</h2>
+
+<p>This is a delta specification, meaning that it currently contains
+only the differences from CSS Animations Level 1 [[!CSS3-ANIMATIONS]].
+Once the Level 1 specification is closer to complete, it will be merged
+with the additions here into a complete level 2 specification.</p>
+
+<h2 id="events">Animation Events</h2>
+
+<h3 id="event-animationevent">Types of <code>AnimationEvent</code></h3>
+
+	<p>The <em>additional</em> types of animation events that can occur are:</p>
+
+	<dl dfn-type=event dfn-for=animationevent>
+		<dt><dfn>animationcancel</dfn>
+		<dd>
+			The <a idl>animationcancel</a> event occurs when the animation stops running in a way that does not fire an <a idl>animationend</a> event, such as a change in the 'animation-name' that removes the animation, or the animating element or one of its ancestors becoming ''display:none''.
+			<ul>
+				<li>Bubbles: Yes</li>
+				<li>Cancelable: No</li>
+				<li>Context Info: animationName, elapsedTime, pseudoElement</li>
+			</ul>
+	</dl>
+