[css-animations] Update definition of CSSKeyframeRule

* Include the [SameObject, PutForwards=cssText] annotations on the style
  attribute in keeping with the other style attributes in CSSOM etc.

* Specify that when an invalid keyText is set, not only is an exception thrown
  but the original value is unchanged.

* Define the various properties of the object returned by the style attribute
  including the:
    readonly flag
    declarations
    parent CSS rule
    owner node

This closes w3c#906.

Author: birtles

css-animations/Overview.bs

@@ -553,9 +553,9 @@ The 'animation-duration' property</h3>
 			''both'', the first frame of the animation, as defined by
 			'animation-direction', will be displayed during the
 			'animation-delay'. Then the last frame of the animation,
-			as defined by 'animation-direction', will be displayed if
-			'animation-fill-mode' is set to ''forwards'' or ''both''.
-			If 'animation-fill-mode' is set to ''animation-fill-mode/none''
+      as defined by 'animation-direction', will be displayed if
+      'animation-fill-mode' is set to ''forwards'' or ''both''.  If
+      'animation-fill-mode' is set to ''animation-fill-mode/none''
 			then the animation has no visible effect.
 		</dd>
 	</dl>
@@ -1091,29 +1091,46 @@ IDL Definition</h4>
 
 	<pre class="idl">
 	interface CSSKeyframeRule : CSSRule {
-	           attribute DOMString           keyText;
-	  readonly attribute CSSStyleDeclaration style;
+    attribute DOMString keyText;
+    [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
 	};
 	</pre>
 
 <h4 id="interface-csskeyframerule-attributes">
 Attributes</h4>
 
-	<dl dfn-type=attribute dfn-for=CSSKeyframeRule>
-
-		<dt><dfn attribute for="CSSKeyframeRule">keyText</dfn>
-		<dd>
-			This attribute represents the keyframe selector as a comma-separated list of
-			percentage values. The ''from'' and ''to'' keywords map to 0% and 100%,
-			respectively.
-
-            If <a attribute for="CSSKeyframeRule">keyText</a> is updated with an invalid keyframe selector,
-            a <a spec="webidl" exception>SyntaxError</a> exception must be thrown.
-
-		<dt><dfn>style</dfn>
-		<dd>
-			This attribute represents the style associated with this keyframe.
-	</dl>
+  <dl dfn-type=attribute dfn-for=CSSKeyframeRule>
+
+    <dt><dfn>keyText</dfn>
+    <dd>
+      This attribute represents the keyframe selector as a comma-separated list
+      of percentage values. The ''from'' and ''to'' keywords map to 0% and 100%,
+      respectively.
+
+      If <a attribute for=CSSKeyframeRule>keyText</a> is updated with an
+      invalid keyframe selector, a <a spec=webidl exception>SyntaxError</a>
+      exception must be thrown and the value of <a attribute
+      for=CSSKeyframeRule>keyText</a> must remain unchanged.
+    </dd>
+
+    <dt><dfn>style</dfn>
+    <dd>
+      Must return a {{CSSStyleDeclaration}} object for the
+      keyframe rule, with the following properties:
+
+      <dl>
+        <dt><a spec=cssom for=CSSStyleDeclaration>readonly flag</a></dt>
+        <dd>Unset.</dd>
+        <dt><a spec=cssom for=CSSStyleDeclaration>declarations</a></dt>
+        <dd>The declared declarations in the rule, in <a spec=cssom>specified
+            order</a>.</dd>
+        <dt><a spec=cssom for=CSSStyleDeclaration>parent CSS rule</a></dt>
+        <dd>The <a>context object</a> (i.e. this {{CSSKeyframeRule}}).</dd>
+        <dt><a spec=cssom for=CSSStyleDeclaration>owner node</a></dt>
+        <dd>Null.</dd>
+      </dl>
+    </dd>
+  </dl>
 
 <h3 id="interface-csskeyframesrule">
 The <code>CSSKeyframesRule</code> Interface</h3>