[css-values] Move the calc()-unwrapping to computed value, not serialization, so it doesn't happen when serializing specified values.

Author: tabatkins

css-values/Overview.bs

@@ -1506,6 +1506,13 @@ Computed Value</h4>
 	as either a number
 	or a tuple of a dimension and a percentage.
 
+	If this resolution process results in only a single value
+	(one <<number>>, one <<dimension>>, or one <<percentage>>),
+	the computed value is just that value,
+	not a ''calc()'' containing that value.
+	If this value is outside the allowed range for the context,
+	it must be clamped to the nearest allowed value.
+
 	<div class='example'>
 		For example, 'background-position' has special behavior for percentage values, different from lengths:
 
@@ -1583,12 +1590,7 @@ Serialization</h4>
 		The result must be a summation of unique incompatible units.
 		(Terms with a value of zero <strong>must</strong> be preserved in this summation.)
 
-	2. If the result is a single 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,
+	2. Serialize as a ''calc()'' containing the summation,
 		with the units ordered <a>ASCII case-insensitive</a> alphabetically,
 		numbers coming before all units,
 		and percentages coming after all units.

css-values/issues-cr-2015.bsi

@@ -145,3 +145,11 @@ Comment:  https://lists.w3.org/Archives/Public/www-style/2016Feb/0184.html
 Comment:  https://lists.w3.org/Archives/Public/www-style/2016Mar/0178.html
 Response:
 Open:
+----
+Issue 18.
+Summary:  Implementations only drop calc() from "simple values" at computed-value time
+From:     David Baron
+Comment:  https://lists.w3.org/Archives/Public/www-style/2016Feb/0184.html
+Response: https://lists.w3.org/Archives/Public/www-style/2016Mar/0178.html
+Closed:   Accepted
+Resolved: