make the definition a bit more clear

Author: annevk

cssom/Overview.html

@@ -1858,11 +1858,11 @@ <h4 id=the-cssstyledeclaration><span class=secno>3.6.2. </span>The <code><a
      <code>null</code> or the empty string abort this algorithm.
 
    <li>
-    <p>If <var title="">value</var> <a href="#parse" title="CSS value
-     parsing">cannot be parsed</a> into a legal value abort this algorithm.</p>
+    <p>If <a href="#parse" title="CSS value parsing">parsing the <var
+     title="">value</var></a> returns "null" abort this algorithm.</p>
 
     <p class=note><var title="">value</var> can not include
-     <code>!important</code> or <code>;</code>.</p>
+     <code>!important</code>.</p>
 
    <li>
     <p>Finally, set <var title="">property</var> to <var title="">value</var>

cssom/Overview.src.html

@@ -1479,11 +1479,11 @@ <h4>The <code>CSSStyleDeclaration</code> Interface</h4>
    <code>null</code> or the empty string abort this algorithm.</p></li>
 
    <li>
-    <p>If <var title="">value</var> <span title="CSS value parsing">cannot be
-    parsed</span> into a legal value abort this algorithm.</p>
-    
+    <p>If <span title="CSS value parsing">parsing the
+    <var title="">value</var></span> returns "null" abort this algorithm.</p>
+
     <p class="note"><var title="">value</var> can not include
-    <code>!important</code> or <code>;</code>.</p>
+    <code>!important</code>.</p>
    </li>
 
    <li><p>Finally, set <var title="">property</var> to <var title="">value</var>