[css-animations] Take a first pass at updating animation start time prose to account for missing @Keyframes

Author: Sylvain Galineau

css-animations/Overview.bs

@@ -96,12 +96,17 @@ Animations</h2>
 	delay). During an animation, the computed style is derived from the
 	animated value.
 
-	The start time of an animation is the time at which the style that defines
-	the animation is resolved. An animation specified by dynamically modifying
-	the element's style will start when this style is resolved; that may be
-	immediately in the case of a pseudo style rule such as hover, or may be
-	when the scripting engine returns control to the browser (in the case of
-	style applied by script).
+	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
+	@keyframes rule does not yet exist, the animation cannot start; the 
+	animation will start from the beginning as soon as a matching @keyframes 
+	rule can be resolved. An animation specified by dynamically modifying the 
+	element's style will start when this style is resolved; that may be 
+	immediately in the case of a pseudo style rule such as hover, or may be 
+	when the scripting engine returns control to the browser (in the case of 
+	style applied by script). 
+	 
 
 	An animation applies to an element if its name appears as one of the
 	identifiers in the computed value of the 'animation-name' property and the