[css-color-4] Clarify serializing opacity values with calc. #10426

Author: svgeesus

css-color-4/Overview.bs

@@ -6763,14 +6763,35 @@ Serializing other colors</h3>
 
 	The serialized form of ''currentColor'' is the string "currentcolor".
 
+<!-- Big Text: Opacity Serial 
+
+ ███▌  ████▌   ███▌   ███▌  ████ █████▌ █   ▐▌      ███▌  █████▌ ████▌  ████  ███▌  █▌   
+█▌  █▌ █▌  █▌ ▐█ ▐█  █▌  █▌  ▐▌    █▌   ▐▌  █      █▌  █▌ █▌     █▌  █▌  ▐▌  ▐█ ▐█  █▌   
+█▌  █▌ █▌  █▌ █▌  █▌ █▌      ▐▌    █▌    █ ▐▌      █▌     █▌     █▌  █▌  ▐▌  █▌  █▌ █▌   
+█▌  █▌ ████▌  █▌  █▌ █▌      ▐▌    █▌    ▐▌█        ███▌  ████   ████▌   ▐▌  █▌  █▌ █▌   
+█▌  █▌ █▌     █████▌ █▌      ▐▌    █▌     █▌           █▌ █▌     █▌▐█    ▐▌  █████▌ █▌   
+█▌  █▌ █▌     █▌  █▌ █▌  █▌  ▐▌    █▌     █▌       █▌  █▌ █▌     █▌ ▐█   ▐▌  █▌  █▌ █▌   
+ ███▌  █▌     █▌  █▌  ███▌  ████   █▌     █▌        ███▌  █████▌ █▌  █▌ ████ █▌  █▌ █████
+-->
 
 <h2 id='serializing-opacity-values'>
 Serializing <<opacity-value>></h2>
 
 	This applies to the ''opacity'' property.
 
-	The specified given opacity value is serialized
-	as a <<number>>, not a <<percentage>>.
+	If the specified value for an opacity value
+	matches a literal <<percentage-token>>
+	(i.e. does not use ''calc()'')
+	it should be serialized as the equivalent
+	<<number>> (0% maps to 0, 100% maps to 1) value.
+	value. 
+	Otherwise, the specified value 
+	for an opacity value 
+	should serialize using 
+	the standard serialization for the grammar.
+
+	<!-- The specified given opacity value is serialized
+	as a <<number>>, not a <<percentage>>. -->
 
 	This <<number>> value is expressed in base ten,
 	with the "." character as decimal separator.