[css3-animations] Require one valid keyframe rule to run an animation; add a note on the validity of empty keyframes rules (bug 15251)

Author: Sylvain Galineau

css3-animations/Overview.html

@@ -437,6 +437,10 @@ <h2 id=keyframes><span class=secno>3. </span> Keyframes</h2>
    percentage values or values higher than 100%, then the keyframe will be
    ignored.
 
+  <p> At least one valid keyframe rule must be specified in order for the
+   animation to run. This requirement does not include rules constructed by
+   the user agent.
+
   <p> The <dfn id=keyframe-declaration-block>keyframe declaration block</dfn>
    for a keyframe rule consists of properties and values. Properties that are
    unable to be animated are ignored in these rules, with the exception of
@@ -455,6 +459,10 @@ <h2 id=keyframes><span class=secno>3. </span> Keyframes</h2>
    not cascade; therefore, an animation will never derive keyframes from more
    than one @keyframes rule.
 
+  <p class=note> Note that since empty @keyframes rule are valid, they may
+   hide the keyframes of those preceding animation definitions with a
+   matching name.
+
   <p> To determine the set of keyframes, all of the values in the selectors
    are sorted in increasing order by time. If there are any duplicates, then
    the last keyframe specified inside the @keyframes rule will be used to

css3-animations/Overview.src.html

@@ -266,6 +266,10 @@ <h2 id="keyframes">
 		negative percentage values or
 		values higher than 100%,
 		then the keyframe will be ignored.
+
+	<p>
+		At least one valid keyframe rule must be specified in order for the animation
+		to run. This requirement does not include rules constructed by the user agent.
 
 	<p>
 		The <dfn>keyframe declaration block</dfn> for a keyframe rule 
@@ -283,6 +287,10 @@ <h2 id="keyframes">
 		that matches the name of the animation specified by the 'animation-name' property. 
 		@keyframes rules do not cascade; 
 		therefore, an animation will never derive keyframes from more than one @keyframes rule.
+
+	<p class='note'>
+		Note that since empty @keyframes rule are valid, they may hide the keyframes of 
+		those preceding animation definitions with a matching name.
 
 	<p>
 		To determine the set of keyframes,