Add note about mod with units on both sides, for future reference.

Author: tabatkins

css3-values/Overview.html

@@ -1331,7 +1331,8 @@ <h3 id=calc><span class=secno>9.1. </span> Mathematical Expressions:
    class=css>-3</code>&rsquo; or &lsquo;<code class=css>2</code>&rsquo;?</p>
   <!-- http://lists.w3.org/Archives/Public/www-style/2010May/0001.html
 	This has notes on how we should handle things when calc() is extended to 
-	handle unit mult/div. -->
+	handle unit mult/div. Related to this is <unit>mod<unit>, which can return
+	0 and thus introduce computed-time division-by-zero. -->
 
   <div class=example>
    <pre>
@@ -1433,8 +1434,8 @@ <h3 id=calc><span class=secno>9.1. </span> Mathematical Expressions:
 
    <li>At &lsquo;<code class=css>/</code>&rsquo; or &lsquo;<code
     class=css>mod</code>&rsquo;, check that the right side is &lsquo;<code
-    class=css>&lt;number></code>&rsquo;; resolve to the type of the left side
-    
+    class=css>&lt;number></code>&rsquo;; resolve to the type of the left
+    side.
   </ul>
 
   <p>If an operator does not pass the above checks, the expression is

css3-values/Overview.src.html

@@ -890,7 +890,8 @@ <h3 id="calc">
 
 	<!-- http://lists.w3.org/Archives/Public/www-style/2010May/0001.html
 	This has notes on how we should handle things when calc() is extended to 
-	handle unit mult/div. -->
+	handle unit mult/div. Related to this is <unit>mod<unit>, which can return
+	0 and thus introduce computed-time division-by-zero. -->
 
 	<div class="example">
 		<pre>
@@ -979,7 +980,7 @@ <h3 id="calc">
 		resolve to the type of the other side</li>
 
 		<li>At ''/'' or ''mod'', check that the right side is ''&lt;number>'';
-		   resolve to the type of the left side
+		resolve to the type of the left side.</li>
 	</ul>
 
 	<p>If an operator does not pass the above checks, the expression is invalid.