Define what it means for a property to not accept a value. Fixes #147.

Author: tabatkins

css-typed-om/Overview.bs

@@ -154,14 +154,14 @@ future without breaking code that relies on calling
 
 	6. For each |value| in |values|:
 		: If |value| is a {{CSSStyleValue}},
-		::	If |value|’s type is not one that |property| is defined to accept,
+		:: If |value| does not match the grammar of a <a>list-valued property iteration</a> of |property|,
 			<a>throw</a> a {{TypeError}} and exit this algorithm.
 			Otherwise, append |value| to the end of |values to append|.
 		: If |value| is a {{DOMString}},
 		::	<a>Parse a CSSStyleValue</a> with property |property| and value |value|.
 			If the result is null,
 			<a>throw</a> a {{TypeError}} and exit this algorithm.
-			Otherwise, append the result to the end of |values to append|.
+			Otherwise, append each <a>list-valued property iteration</a> in the result to the end of |values to append|.
 
 	7. Append |values to append| to the end of |entry|’s list.
 </div>