added division by zero restriction

Author: howcom

css3-values/Overview.src.html

@@ -542,17 +542,27 @@ <h4>The 'calc', 'min' and 'max' functions</h4>
 </pre>
 -->
 
+<p>Division by zero is a parse error.
+
+<!--
+<p>Values inside a calc() expression must all be either (a)
+numbers or (b) legal values for the property, or values that
+would be legal if it weren't for range restrictions (such
+as the restriction that 'width' accepts only positive values).
+-->
 
 <p>The semantic constraints are as follows: The context of the
-expression imposes a target type, which is one of length, frequency,
-angle, time, or number. Any PERCENTAGE token in the expression also
-has that type. The NUMBER tokens obviously are of type number, and the
-DIMENSION tokens have types that depend on their units (cm, px, deg,
+expression imposes a <em>target type</em>, which is one of length, frequency,
+angle, time, or number. Any PERCENTAGE token in the expression has the
+target type. The NUMBER tokens are of type number, and the DIMENSION
+tokens have types of their units ('cm' is length, 'deg' is angle
 etc.). At each operator, the types of the left and right side have to
-be compared and, if compatible, they yield a result type, roughly as
+be compared and, if compatible, they yield a <em>result type</em>, roughly as
 follows (the following ignores precedence rules on the operators for
 simplicity):
 
+
+
 <ol>
 <li>At ",", "+", "-":<br>
    check: both sides have the same type<br>
@@ -565,7 +575,6 @@ <h4>The 'calc', 'min' and 'max' functions</h4>
 </ol>
 
 
-
 <h3>&lt;string&gt;</h3>
 
 <p>Strings are denoted by <dfn>&lt;string&gt;</dfn> in the value