[web-animations-1] Consistently refer to keyframe-specific composite operations as having a null value

This closes w3c#2289.

Author: birtles

web-animations-1/Overview.bs

@@ -3245,14 +3245,14 @@ is specified and the <em>next</em> keyframe in the list.
 The <a>timing function</a> specified on the last keyframe in the
 list is never applied.
 
-Each <a>keyframe</a> may also have a <dfn>keyframe-specific composite
-operation</dfn> that is applied to all values
+Each <a>keyframe</a> may has a <dfn>keyframe-specific composite
+operation</dfn> that, if not null,  is applied to all values
 specified in that <a>keyframe</a>.
 The possible operations and their meanings are identical to those defined
 for the <a>composite operation</a> associated with the <a>keyframe effect</a>
 as a whole in [[#effect-composition]].
-If no <a>keyframe-specific composite operation</a> is specified for
-a <a>keyframe</a>, the <a>composite operation</a> specified for the
+If the <a>keyframe-specific composite operation</a> for a <a>keyframe</a>
+is null, the <a>composite operation</a> specified for the
 <a>keyframe effect</a> as a whole is used for values specified in that keyframe.
 
 <h4 id="calculating-computed-keyframes">Calculating computed keyframes</h4>
@@ -4864,16 +4864,16 @@ interface KeyframeEffect : KeyframeEffectReadOnly {
             };
             </pre>
 
-        1.  Set {{BaseComputedKeyframe/offset}},
+        1.  Set the {{BaseComputedKeyframe/offset}},
             {{BaseComputedKeyframe/computedOffset}},
-            {{BaseComputedKeyframe/easing}} members of
-            <var>output keyframe</var> to the respective values
+            {{BaseComputedKeyframe/easing}}, and
+            {{BaseComputedKeyframe/composite}} members of
+            <var>output keyframe</var> to the respective
             <a>keyframe offset</a>,
-            <a>computed keyframe offset</a>, and
-            keyframe-specific <a>timing function</a>
-            of <var>keyframe</var>.
-        1.  If <var>keyframe</var> has a <a>keyframe-specific composite
-            operation</a>, set {{BaseComputedKeyframe/composite}} to that value.
+            <a>computed keyframe offset</a>,
+            keyframe-specific <a>timing function</a>, and
+            <a>keyframe-specific composite operation</a>
+            values of <var>keyframe</var>.
         1.  For each animation property-value pair specified on
             <var>keyframe</var>, <var>declaration</var>, perform the following
             steps:
@@ -5576,8 +5576,8 @@ dictionary KeyframeEffectOptions : AnimationEffectTimingProperties {
 ::  The <a>composite operation</a> used to composite this
     animation with the <a>effect stack</a>, as specified by one
     of the <a>CompositeOperation</a> enumeration values.
-    This is used for all <a>keyframes</a> that do not specify
-    a <a>keyframe-specific composite operation</a>.
+    This is used for all <a>keyframes</a> that specify
+    a null <a>keyframe-specific composite operation</a>.
 
 </div>