[css-counter-styles] Abstract the list of non-overrideable counter-style names and use it directly. w3c#8186

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -163,13 +163,15 @@ Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
 
 	<dfn>&lt;counter-style-name></dfn> is a <<custom-ident>>
 	that is not an <a>ASCII case-insensitive</a> match for ''list-style-type/none''.
+	When used here, to define a counter style,
+	it also cannot be any of the [=non-overrideable counter-style names=]
+	(in other uses that merely reference a counter style,
+	such as the ''extend'' system,
+	these are allowed).
 	The <<counter-style-name>> is a [=tree-scoped name=].
 
-	The keywords ''decimal'', ''disc'', ''square'', ''circle'', ''disclosure-open'', and ''disclosure-closed''
-	are valid <<counter-style-name>>s,
-	but are invalid when used here to name a counter style rule;
-	doing so makes the rule invalid.
-	(They can be used in other contexts, such as in the ''extend'' system.)
+	The <dfn>non-overrideable counter-style names</dfn> are
+	the keywords ''decimal'', ''disc'', ''square'', ''circle'', ''disclosure-open'', and ''disclosure-closed''.
 
 	Note: Note that <<custom-ident>> also automatically excludes the <a spec=css-values>CSS-wide keywords</a>.
 	In addition, some names, like ''inside'',
@@ -2574,14 +2576,16 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 
 			On setting the <var>name</var> attribute, run the following steps:
 
-			1. If the value is an <a>ASCII case-insensitive</a> match
+			1. If the value is an [=ASCII case-insensitive=] match
+				for "none"
+				or one of the [=non-overrideable counter-style names=],
+				do nothing and return.
+
+			2. If the value is an <a>ASCII case-insensitive</a> match
 				for any of the predefined <a>counter styles</a>,
 				lowercase it.
 
-			2. If the value is not "decimal", "disc", or "none",
-				replace the associated rule's name with an <a>identifier</a> equal to the value.
-
-			3. Otherwise, do nothing.
+			3. Replace the associated rule's name with an <a>identifier</a> equal to the value.
 
 		<dt><dfn>system</dfn>
 		<dt><dfn>symbols</dfn>