[css-values-4] Fix min()/max() partial simplification to not imply that %s will always block simplification, and add a Note making the % behavior more explicit. w3c#6298

Author: tabatkins

css-values-4/Overview.bs

@@ -4189,11 +4189,47 @@ Simplification</h4>
 		3. If |root| is an [=operator node=]
 			that's not one of the [=calc-operator nodes=],
 			and all of its children are numeric values
-			with enough information to computed the operation |root| represents,
+			with enough information to compute the operation |root| represents,
 			return the result of running |root|'s operation
 			using its children,
 			expressed in the result's [=canonical unit=].
 
+			<div class=note>
+
+				If a percentage is left at this point,
+				it will <em>usually</em> block simplification of the node,
+				since it needs to be resolved against another value
+				using information not currently available.
+				(Otherwise, it would have been converted to a different value in an earlier step.)
+				This includes operations such as "min",
+				since percentages might resolve against a negative basis,
+				and thus end up with an opposite comparative relationship
+				than the raw percentage value would seem to indicate.
+
+				However, "raw" percentages--
+				ones which do not resolve against another value,
+				such as in 'opacity'--
+				might not block simplification.
+			</div>
+
+
+		8. If |root| is a Min or Max node,
+			attempt to <em>partially</em> simplify it:
+
+			1. [=list/For each=] node |child| of |root|’s children:
+
+				If |child| is a numeric value
+				with enough information to compare magnitudes with another child of the same unit
+				(see note in previous step),
+				and there are other children of |root|
+				that are numeric values with the same unit,
+				combine all such children
+				with the appropriate operator per |root|,
+				and replace |child| with the result,
+				removing all other child nodes involved.
+
+			2. Return |root|.
+
 		4. If |root| is a Negate node:
 
 			1. If |root|'s child is a numeric value,
@@ -4267,23 +4303,6 @@ Simplification</h4>
 
 			5. Return |root|.
 
-		8. If |root| is a Min or Max node:
-
-			1. [=list/For each=] node |child| of |root|’s children:
-
-				If |child| is a numeric value
-				with a unit other than "percent",
-				and there are other children of |root|
-				that are numeric values with the same unit,
-				combine all such children
-				with the appropriate operator per |root|,
-				and replace |child| with the result,
-				removing all other child nodes involved.
-
-			2. If |root| now has only a single child,
-				return that child.
-				Otherwise, return |root|.
-
 	</div>
 
 <h3 id='calc-computed-value'>