[css-values-4] Fix <calc-constant> ref to be <calc-keyword>, and allow for keywords to resolve to non-numeric values

Author: tabatkins

css-values-4/Overview.bs

@@ -4181,8 +4181,8 @@ Type Checking</h3>
 				:: the [=CSSNumericValue/type=] is «[ "resolution" → 1 ]»
 				: <<flex>>
 				:: the [=CSSNumericValue/type=] is «[ "flex" → 1 ]»
-				: <<calc-constant>>
-				:: the [=CSSNumericValue/type=] is «[ ]» (empty map)
+				: <<calc-keyword>>
+				:: the [=CSSNumericValue/type=] is as defined by the keyword
 				: <<percentage>>
 				::
 					If, in the context in which the [=math function=]
@@ -4529,8 +4529,9 @@ Simplification</h4>
 				do so,
 				and return the value.
 
-			3. If |root| is a <<calc-constant>>,
-				return its numeric value.
+			3. If |root| is a <<calc-keyword>> that can be resolved,
+				return what it resolves to,
+				[=simplified=].
 
 			4. Otherwise, return |root|.