[css-transitions-2] Specify interaction between running transitions and Web Animations API

Fixes w3c#3233.

The mention of `display:none` handling (the "not rendered" part) is
intended to address issue w3c#2948 and should ultimately be integrated into
level 1.

Author: birtles

css-transitions-2/Overview.bs

@@ -40,6 +40,7 @@ urlPrefix: https://drafts.csswg.org/web-animations/; type: dfn; spec: web-animat
     text: animation effect
     text: animation class
     text: animation playback rate
+    text: cancel an animation
     text: current iteration
     text: current time
     text: fill mode
@@ -53,9 +54,12 @@ urlPrefix: https://drafts.csswg.org/web-animations/; type: dfn; spec: web-animat
     text: target effect
     text: target effect end
     text: unresolved
+urlPrefix: https://html.spec.whatwg.org/multipage/browsers.html; type: dfn; spec: html
+    text: being rendered
 </pre>
 <pre class=link-defaults>
 spec:css-transitions-1; type:value; text:all
+spec:css-transitions-1; type:dfn; text:cancel
 </pre>
 
 <h2 id="delta">Delta specification</h2>
@@ -81,6 +85,25 @@ the reversing shortening factor.
 
 # Starting of transitions # {#starting}
 
+The set of [=running transitions=] includes
+only those transitions that have an [=owning element=].
+
+Whenever an implementation is required to [=cancel=] a transition,
+it must disassociate the transition from its [=owning element=]
+and run the procedure to [=cancel an animation=] on the transition.
+
+Although not explicitly defined by level 1 of this specification,
+in addition to [=cancel|cancelling=] transitions on elements that
+are no longer [=connected=],
+implementations must also [=cancel=] any running transitions
+on elements that are no longer [=being rendered=] and
+remove transitions on them from the set of [=completed transitions=].
+
+Note that calling the {{Animation/cancel()}} method
+on the {{CSSTransition}} object representing a [=running transition=]
+does <em>not</em> cause the transition to be disassociated
+from its [=owning element=].
+
 ## The current transition generation ## {#current-transition-generation-section}
 
 Associated with each top-level browsing context is a <dfn>current transition
@@ -104,15 +127,8 @@ The <dfn>owning element</dfn> of a transition refers to the element or
 pseudo-element to which the 'transition-property' property was applied that
 generated the animation.
 
-If a transition generated using the markup defined in this specification is
-later disassociated from that markup because it is cancelled or replaced by
-a newer transition, the animation is disassociated from its <a>owning
-element</a> (that is, it has no <a>owning element</a> from that point
-forwards).
-
-Issue: Define the above more precisely once we rewrite firing of transitions
-in terms of Web Animations concepts (specifically when we spell out when we
-cancel an animation).
+A transition may be disassociated from its [=owning element=]
+such that it has no [=owning element=].
 
 # Application of transitions # {#application}