[css-animations] Define that animations behave as if will-change was set on animated properties while running/active/delayed

Author: birtles

css-animations/Overview.bs

@@ -105,18 +105,25 @@ Animations</h2>
 	animations override all normal rules, but are overridden by !important
 	rules.
 
-	If at one point in time there are multiple animations specifying behavior
+	If at some point in time there are multiple animations specifying behavior
 	for the same property, the animation which occurs last in the value
 	of 'animation-name' will override the other animations at that point.
 
-	An animation does not affect the computed value before the application of
-	the animation, before the animation delay has expired, and after the end of
-	the animation.
+	An animation does not affect the computed value before the application of the
+	animation (that is, when the 'animation-name' property is set on an element)
+	or after it is removed. Furthermore, typically an animation does not affect
+	the computed value before the animation delay has expired or after the end of
+	the animation, but may do so depending on the 'animation-fill-mode' property.
 
 	While running, the animation computes the value of those properties
 	it animates. Other values may take precedence over the animated value
 	according to the CSS cascade ([[!CSS3CASCADE]]).
 
+	While an animation is applied but has not finished, or has finished but has
+	an 'animation-fill-mode' of ''forwards'' or ''both'', the user agent must act
+	as if the 'will-change' property ([[!css-will-change-1]]) on the element
+	additionally includes all the properties animated by the animation.
+
 	The start time of an animation is the time at which the style applying
 	the animation and the corresponding @keyframes rule are both resolved.
 	If an animation is specified for an element but the corresponding