[css-values-4] Issue w3c#250 - Allow <percentage> to be its own type, for some hypothetical future use.

Author: fantasai

css-values/Overview.bs

@@ -1470,12 +1470,12 @@ Mathematical Expressions: ''calc()''</h3>
 	<<frequency>>,
 	<<angle>>,
 	<<time>>,
+	<<percentage>>,
 	<<number>>, or
 	<<integer>>
 	values are allowed.
-	Components of a ''calc()'' expression can be literal values,
-	''attr()'' or ''calc()'' expressions,
-	or <<percentage>> values that resolve to one of the preceding types.
+	Components of a ''calc()'' expression can be literal values or
+	''attr()'' or ''calc()'' expressions.
 
 	<div class="example">
 		<pre>
@@ -1566,6 +1566,7 @@ Type Checking</h4>
 	<<frequency>>,
 	<<angle>>,
 	<<time>>,
+	<<percentage>>,
 	<<number>>, or
 	<<integer>>.
 	The <a>resolved type</a> must be valid for where the expression is placed;
@@ -1577,8 +1578,13 @@ Type Checking</h4>
 	An ''attr()'' expression's type is given by its <<type-or-unit>> argument.
 
 	If percentages are accepted in the context in which the expression is placed,
-	a <<percentage-token>> has the type of the value that percentages are relative to;
-	otherwise, a math expression containing percentages is invalid.
+	a <<percentage-token>> has the type of the value that percentages are relative to.
+	For example, in the 'width' property, percentages have the <<length>> type,
+	and in 'opacity' they have the <<number>> type.
+	A percentage only has the <<percentage>> type if in that context
+	<<percentage>> values are not used-value compatible with any other type.
+	If percentages are not normally allowed in place of the ''calc()'',
+	then a ''calc()'' expression containing percentages is invalid in that context.
 
 	Operators form sub-expressions, which gain types based on their arguments.
 	To make expressions simpler,