Make processing keyframe procedure handle nulls

As reported:
http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0064.html

Overview.src.html

@@ -6381,13 +6381,15 @@ <h4 id="processing-a-keyframe-object">Processing a <code>Keyframe</code> object<
 
     :   If <var>property</var> is a case-sensitive match for
         the string "offset",</dt>
-    ::  Set the <code>offset</code> member of <var>keyframe
-        result</var> to the result of applying the <a
+    ::  Let <var>offset value</var> be the result of calling the \[[Get]]
+        internal method on <var>keyframe input</var> with property name
+        "offset".
+        If <var>offset value</var> is null, set the <code>offset</code>
+        member of <var>keyframe result</var> to null.
+        Otherwise, set it to the result of applying the <a
         href="http://www.w3.org/TR/WebIDL/#es-double">procedure for
         converting an ECMAScript value into an IDL double</a>
-        [[!WEBIDL]] to the result of calling the \[[Get]]
-        internal method on <var>keyframe input</var> with property
-        name "offset".
+        [[!WEBIDL]] to <var>offset value</a>.
 
     :   If <var>property</var> is a case-sensitive match for
         the string "easing",</dt>
@@ -6410,14 +6412,16 @@ <h4 id="processing-a-keyframe-object">Processing a <code>Keyframe</code> object<
 
     :   If <var>property</var> is a case-sensitive match for
         the string "composite",</dt>
-    ::  Set the <code>composite</code> member of <var>keyframe
-        result</var> to the result applying <a
-        href="http://www.w3.org/TR/WebIDL/#es-enumeration">the
-        procedure for converting an ECMAScript value to an IDL
-        enumeration type</a> [[!WEBIDL]] with
-        <a>CompositeOperation</a> as the enumeration type, to
-        the result of calling the \[[Get]] internal method on
-        <var>keyframe input</var> with property name "composite".
+    ::  Let <var>composite value</var> be the result of calling the \[[Get]]
+        internal method on <var>keyframe input</var> with property name
+        "composite".
+        If <var>composite value</var> is null, set the <code>composite</code>
+        member of <var>keyframe result</var> to null.
+        Otherwise, set it to the result of applying the <a
+        href="http://www.w3.org/TR/WebIDL/#es-enumeration">procedure for
+        converting an ECMAScript value to an IDL enumeration type</a>
+        [[!WEBIDL]] with <a>CompositeOperation</a> as the enumeration type,
+        to <var>composite value</a>.
 
     :   Otherwise, if <var>property</var> also exists in
         <var>supported properties</var> based on a case-sensitive