[css-values] Clamp calc() values at both computed and used value time…

… per WG resolution. #434

css-values-3/Overview.bs

@@ -1740,8 +1740,12 @@ Range Checking</h4>
 
 	Parse-time range-checking of values is not performed within ''calc()'',
 	and therefore out-of-range values do not cause the declaration to become invalid.
-	However, the used value resulting from an expression
+	However, the value resulting from an expression
 	must be clamped to the range allowed in the target context.
+	Clamping is performed on computed values to the extent possible,
+	and also on used values
+	if computation was unable to sufficiently simplify the expression
+	to allow range-checking.
 
 	Note: This requires all contexts accepting ''calc()''
 	to define their allowable values as a closed (not open) interval.
@@ -2140,6 +2144,9 @@ Changes</h2>
 		<li>Reverted previous change to allow zero angles to drop their unit;
 			this will instead be special-cased where needed for backwards-compatibility.
 			(See <a href="https://lists.w3.org/Archives/Public/www-style/2017Apr/0027.html">discussion</a>)
+		<li>Defined that range checking, and any resulting clamping, of ''calc()'' values
+			is performed both at computed time and at used time.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/434">Issue #434</a>)
 		<li>Fixed grammar error that disallowed numeric expressions as denominators in ''calc()''.
 			(<a href="https://drafts.csswg.org/css-values-3/issues-cr-2016#issue-12">Issue 12</a>)
 		<li>Defined handling of font-relative units outside the context of an element.

css-values-4/Overview.bs

@@ -1880,8 +1880,12 @@ Range Checking</h4>
 
 	Parse-time range-checking of values is not performed within [=math functions=],
 	and therefore out-of-range values do not cause the declaration to become invalid.
-	However, the used value resulting from an expression
+	However, the value resulting from an expression
 	must be clamped to the range allowed in the target context.
+	Clamping is performed on computed values to the extent possible,
+	and also on used values
+	if computation was unable to sufficiently simplify the expression
+	to allow range-checking.
 
 	Note: This requires all contexts accepting ''calc()''
 	to define their allowable values as a closed (not open) interval.