Make it clear that dividing by an expression that evaluates to 0 is invalid in calc().

Author: tabatkins

css3-values/Overview.html

@@ -1439,7 +1439,10 @@ <h3 id=calc><span class=secno>9.1. </span> Mathematical Expressions:
   </ul>
 
   <p>If an operator does not pass the above checks, the expression is
-   invalid. Also, division by zero is invalid.
+   invalid. Also, division by zero is invalid. This includes both dividing by
+   the literal number zero, as well as any numeric expression that evaluates
+   to zero (as purely-numeric expressions can be evaluated without any
+   additional information at parse time).
 
   <p>The value resulting from an expression must be clamped to the range
    allowed in the target context.

css3-values/Overview.src.html

@@ -984,7 +984,10 @@ <h3 id="calc">
 	</ul>
 
 	<p>If an operator does not pass the above checks, the expression is invalid.
-	Also, division by zero is invalid.</p>
+	Also, division by zero is invalid.  This includes both dividing by the 
+	literal number zero, as well as any numeric expression that evaluates to zero
+	(as purely-numeric expressions can be evaluated without any additional 
+	information at parse time).</p>
 
 	<p>The value resulting from an expression must be clamped to the
 	range allowed in the target context.