[css-values-5] Clarify that random-ua-ident must never be omitted even if the SSP would normally allow it. #13337

Author: tabatkins

css-values-5/Overview.bs

@@ -2412,6 +2412,11 @@ Simplification</h4>
 		before any canonicalization/reordering might occur
 		that could shuffle the values around.
 
+	If the <<random-cache-key>> contains a <<random-ua-ident>>,
+	this must not be omitted in the serialization of the value,
+	even if this would normally be valid per the
+	[=serialize a CSS value|"shortest serialization principle"=].
+
 	<div class=example>
 		For example,
 		specifying ''margin: random(10px, 20px) random(10px, 20px)''