Allow CSSKeyframesRule.name to set reserved values

It is possible to set these values using the quoted string syntax so it
doesn't make sense to prohibit setting these values from the CSSOM.

This closes w3c#801.

Author: birtles

css-animations/Overview.bs

@@ -1143,11 +1143,11 @@ IDL Definition</h4>
 
 	<pre class="idl">
 	interface CSSKeyframesRule : CSSRule {
-	           attribute CSSOMString   name;
+	           attribute CSSOMString name;
 	  readonly attribute CSSRuleList cssRules;
 
-	  void            appendRule(CSSOMString rule);
-	  void            deleteRule(CSSOMString select);
+	  void             appendRule(CSSOMString rule);
+	  void             deleteRule(CSSOMString select);
 	  CSSKeyframeRule? findRule(CSSOMString select);
 	};
 	</pre>
@@ -1161,11 +1161,6 @@ Attributes</h4>
 		<dd>
 			This attribute is the name of the keyframes, used by the 'animation-name' property.
 
-			Setting this property to a value matching a
-			<a href="https://www.w3.org/TR/css3-values/#common-keywords">CSS-wide keyword</a>
-			or any keyword defined for the 'animation-name' property will throw a <a spec="webidl" exception>SyntaxError</a>
-			exception.
-
 		<dt><dfn>cssRules</dfn>
 		<dd>
 			This attribute gives access to the keyframes in the list.