Make parsing fail on invalid types

Author: noamr

css-view-transitions-2/Overview.bs

@@ -2037,11 +2037,14 @@ The [=@view-transition/types=] descriptor
 		: <dfn>none</dfn>
 		:: No transition types are set.
 
-		: <dfn><<custom-ident>>+</dfn>
+		: <dfn><<view-transition-type>>+</dfn>
 		:: Sets the [=ViewTransition/active types=] for the transition to the specified custom identifiers.
-			 The <<custom-ident>> `none`, as well as any <<custom-ident>> starting with `-ua-`, are ignored.
+			 The <<custom-ident>> `none`, as well as any <<custom-ident>> starting with `-ua-`, are considered in.
 	</dl>
 
+	where <<view-transition-type>> is any <<custom-ident>> other than `none`, and not starting with the string "-ua-".
+
+
 	Note: the [=@view-transition/types=] descriptor only applies to the {{Document}} in which it is defined.
 	The author is responsible for using their chosen set of types in both documents.