revised language on operators

Author: howcom

css3-values/Overview.src.html

@@ -521,58 +521,57 @@ <h4>The 'calc', 'min' and 'max' functions</h4>
 <pre>
 S       : calc | min | max;
 calc    : "calc(" S* sum ")" S*;
-min     : "min(" S* sum [ "," S* sum ]* )"" S*;
-max     : "max(" S* sum [ "," S* sum ]* ")" S*;
-sum     : product [ S+ [ "+" | "-" ] S+ product ]*;
-product : unit [[ "*" S* unit ] | [ "/" S* NUMBER ]] *;
-unit    : ["+"|"-"]? [ NUMBER S* | DIMENSION S* | PERCENTAGE S* |
-          min | max | "(" S* sum ")" S* ];
-</pre>
-
-<!--
-<pre>
-S       : calc | min | max;
-calc    : "calc(" S* sum ")" S*;
-min     : "min(" S* sum [ "," S* sum ]* )"" S*;
+min     : "min(" S* sum [ "," S* sum ]* ")" S*;
 max     : "max(" S* sum [ "," S* sum ]* ")" S*;
 sum     : product [ [ "+" | "-" ] S* product ]*;
 product : unit [ [ "*" | "/" | "mod" ] S* unit ]*;
 unit    : ["+"|"-"]? [ NUMBER S* | DIMENSION S* | PERCENTAGE S* |
           min | max | "(" S* sum ")" S* ];
 </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 <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 <em>result type</em>, roughly as
-follows (the following ignores precedence rules on the operators for
-simplicity):
-
 
+<p>The context of the expression imposes a target type, which is one
+of length, frequency, angle, time, or number. NUMBER tokens are of
+type number. DIMENSION tokens have types of their units ('cm' is
+length, 'deg' is angle etc.); any DIMENSION whose type does not match
+the target type is not allowed and must be a parse error. If
+percentages are accepted in that context and convertible to the target
+type, a PERCENTAGE token in the expression has the target type;
+otherwise percentages are not allowed and must be a parse error.
+
+<p>To make expressions simpler, operators have restriction on the
+types they accept. At each operator, the types of the left and right
+side have to be checked for these restrictions. If compatible, they
+return 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>
    return: that type
 <li>At "*":<br>
-  check: at least one side is "number" return: the type of the other side
+   check: at least one side is "number" <br>
+   return: the type of the other side
 <li>At "/":<br> 
    check: right side is "number"<br>
    return: the type of the left side
 </ol>
 
+<p>Division by zero is a parse error.
+
+<p>The value resulting from an expression must be clamped to the
+range allowed in the target context.  
+
+<div class=example>
+These two are equivalentequivalent to 'width: 0px' since widths
+smaller than 0px are not allowed.
+
+<pre>
+width: calc(5px - 10px);
+width: 0px;
+</pre>
+</div>
+
+
 <p>Given the complexities of 'width' and 'height' on table cells and
   table elements, calc() expressions for 'width' and 'height' on table
   columns, table column groups, table rows, table row groups, and