[css-counter-styles-3] Define that counter styles are tree-scoped. w3c#5693

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -160,6 +160,7 @@ 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 "none".
+	The <<counter-style-name>> is a [=tree-scoped name=].
 
 	Additionally, in the context of the prelude of an ''@counter-style'' rule,
 	an <a>ASCII case-insensitive</a> match for "decimal" or "disc" is also an invalid <<counter-style-name>>
@@ -1292,6 +1293,8 @@ Extending 'list-style-type', ''counter()'', and ''counters()''</h2>
 	it must act identically to the ''decimal'' counter style
 	(but does not <em>[=computed value|compute=]</em> to ''decimal'').
 
+	When used in these contexts, a <<counter-style-name>> is a [=tree-scoped reference=].
+
 
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