[css-values] Remove ability for percentages to resolve against numbers. Fixes #1463.

Author: tabatkins

css-values-3/Overview.bs

@@ -1604,14 +1604,30 @@ Type Checking</h4>
 	are valid.
 
 	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.
-	For example, in the 'width' property, percentages have the <<length>> type,
-	and in 'opacity' they have the <<number>> type.
+	and they are defined to be relative to another type besides <<number>>,
+	a <<percentage-token>> is treated as that type.
+	For example, in the 'width' property, percentages have the <<length>> 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.
 
+	Note: Note that <<percentage>>s relative to <<number>>s,
+	such as in 'opacity',
+	are not allowed in ''calc()''.
+	Allowing this would cause significant problems with "unit algebra"
+	(allowing multiplication/division of <<dimension>>s),
+	and in every case so far,
+	doesn't provide any new functionality.
+	(For example, ''opacity: 25%'' is identical to ''opacity: .25'';
+	it's just a trivial syntax transform.)
+
+	Note: Altho there are a few properties in which a bare <<number>>
+	becomes a <<length>> at used-value time
+	(specifically, 'line-height' and 'tab-size'),
+	<<number>>s never become "length-like" in ''calc()''.
+	They always stay as <<number>>s.
+
 	Operators form sub-expressions, which gain types based on their arguments.
 	To make expressions simpler,
 	operators have restrictions on the types they accept.

css-values/Overview.bs

@@ -1691,14 +1691,30 @@ Type Checking</h4>
 	are valid.
 
 	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.
-	For example, in the 'width' property, percentages have the <<length>> type,
-	and in 'opacity' they have the <<number>> type.
+	and they are defined to be relative to another type besides <<number>>,
+	a <<percentage-token>> is treated as that type.
+	For example, in the 'width' property, percentages have the <<length>> 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.
 
+	Note: Note that <<percentage>>s relative to <<number>>s,
+	such as in 'opacity',
+	are not allowed in ''calc()''.
+	Allowing this would cause significant problems with "unit algebra"
+	(allowing multiplication/division of <<dimension>>s),
+	and in every case so far,
+	doesn't provide any new functionality.
+	(For example, ''opacity: 25%'' is identical to ''opacity: .25'';
+	it's just a trivial syntax transform.)
+
+	Note: Altho there are a few properties in which a bare <<number>>
+	becomes a <<length>> at used-value time
+	(specifically, 'line-height' and 'tab-size'),
+	<<number>>s never become "length-like" in ''calc()''.
+	They always stay as <<number>>s.
+
 	Operators form sub-expressions, which gain types based on their arguments.
 	To make expressions simpler,
 	operators have restrictions on the types they accept.