[css-cascade-5] Reserve the CSS-wide keywords for later use in layer names w3c#6323

Author: mirisuzanne

css-cascade-5/Overview.bs

@@ -1349,6 +1349,9 @@ Layer Naming and Nesting</h4>
 		<dfn><<layer-name>></dfn> = <<ident>> [ '.' <<ident>> ]*
 	</pre>
 
+	The <a spec=css-values>CSS-wide keywords</a> are reserved for future use,
+	and cause the rule to be invalid at parse time
+	if used as an <<ident>> in the <<layer-name>>.
 	When multiple identifiers are concatenated with a period,
 	this is a shorthand representing those layers nested in order.