[css-values-3] Specify clamping during serialization.

Author: tabatkins

css-values/Overview.bs

@@ -30,6 +30,7 @@ Ignored Vars: Cn+1, n
 spec: css-backgrounds-3; type: type; text: <position>
 spec: css21; type: property; text: border-collapse
 spec: css-color-4; type: value; text: currentcolor
+spec: css-cascade-4; type: at-rule; text: @import
 </pre>
 <style>
 code, small { white-space: nowrap }
@@ -1572,7 +1573,8 @@ Serialization</h4>
 		(Terms with a value of zero <strong>must</strong> be preserved in this summation.)
 
 	2. If the result is a single value,
-		serialize as that value,
+		clamp the value to the range allowed in the target context,
+		then serialize as that value,
 		discarding the ''calc()'' wrapper.
 
 	3. Otherwise, serialize as a ''calc()'' containing the summation,