[css-transitions-2] Add delta specification with transitionstart and transitioncancel events.

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

Author: dbaron

css-transitions-2/Overview.bs

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+<h1>CSS Transitions Level 2</h1>
+
+<pre class='metadata'>
+Status: ED
+Shortname: css-transitions-2
+Level: 2
+Group: csswg
+ED: http://dev.w3.org/csswg/css-transitions-2/
+Editor: L. David Baron, Mozilla, dbaron@dbaron.org
+Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;product=CSS&amp;component=Transitions&amp;resolution=---
+Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.
+</pre>
+<pre class="link-defaults">
+spec:css-transitions-1; type:dfn; text:cancel
+spec:css-transitions-1; type:dfn; text:running transitions
+</pre>
+
+<h2 id="delta">Delta specification</h2>
+
+	<p>This is a delta specification, meaning that it currently contains
+	only the differences from CSS Transitions Level 1 [[!CSS3-TRANSITIONS]].
+	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="transition-events"><span id="transition-events-">Transition Events</span></h2>
+
+<h3 id="transition-event-types">Types of <code>TransitionEvent</code></h3>
+
+	<p>The <em>additional</em> types of transition events that can occur are:</p>
+
+	<dl dfn-type=event dfn-for=transitionevent>
+		<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>).
+			<ul>
+				<li>Bubbles: Yes</li>
+				<li>Cancelable: No</li>
+				<li>Context Info: propertyName, elapsedTime, pseudoElement</li>
+			</ul>
+		<dt><dfn>transitioncancel</dfn>
+		<dd>
+			The <a idl>transitioncancel</a> event occurs when a transition is <a data-lt="cancel">cancelled</a>.
+			<ul>
+				<li>Bubbles: Yes</li>
+				<li>Cancelable: No</li>
+				<li>Context Info: propertyName, elapsedTime, pseudoElement</li>
+			</ul>
+	</dl>
+