Added an explanator sentence calling out the behavior difference between invalid syntax and invalid variables.

Author: tabatkins

css-variables/Overview.html

@@ -401,7 +401,13 @@ <h3 id=using-invalid-variables><span class=secno>3.1. </span> Using Invalid
 
    <p>the &lt;p> elements will have transparent backgrounds (the initial
     value for &lsquo;<code class=property>background-color</code>&rsquo;),
-    rather than red backgrounds. This is different from the second</p>
+    rather than red backgrounds.</p>
+
+   <p>Note the difference between this and what happens if the author had
+    just written &lsquo;<code class=css>background-color: 20px</code>&rsquo;
+    directly in their stylesheet - that would be a normal syntax error, which
+    would cause the rule to be discarded, so the &lsquo;<code
+    class=css>background-color: red</code>&rsquo; rule would be used instead.</p>
   </div>
 
   <p class=note>The <a href="#invalid-at-computed-value-time"><i>invalid at

css-variables/Overview.src.html

@@ -182,7 +182,9 @@ <h3 id='using-invalid-variables'>
 p { background-color: red; }
 p { background-color: $invalid; }</pre>
 
-		<p>the &lt;p> elements will have transparent backgrounds (the initial value for 'background-color'), rather than red backgrounds.  This is different from the second</p>
+		<p>the &lt;p> elements will have transparent backgrounds (the initial value for 'background-color'), rather than red backgrounds.</p>
+
+		<p>Note the difference between this and what happens if the author had just written ''background-color: 20px'' directly in their stylesheet - that would be a normal syntax error, which would cause the rule to be discarded, so the ''background-color: red'' rule would be used instead.</p>
 	</div>
 
 	<p class='note'>The <i>invalid at computed-value time</i> concept exists because variables can't "fail early" like other syntax errors can, so by the time the user agent realizes a property value is invalid, it's already thrown away the other cascaded values.  I think ''attr()'' needs to rely on it as well, as its behavior is almost identical to variables.</p>