[css-counter-styles-3] Make all the predefined symbolics non-overrideable, and specify that extending them uses the spec version, not the UA exception. #3584

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -162,9 +162,11 @@ Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
 	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>>
-	and makes the ''@counter-style'' rule invalid.
+	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.)
 
 	Note: Note that <<custom-ident>> also automatically excludes the <a spec=css-values>CSS-wide keywords</a>.
 	In addition, some names, like ''inside'',
@@ -701,10 +703,17 @@ Building from Existing Counter Styles: the ''extends'' system
 	any unspecified descriptors must be taken from the extended counter style specified,
 	rather than taking their initial values.
 
-	If a @counter-style uses the ''extends'' system,
+	If a ''@counter-style'' uses the ''extends'' system,
 	it must not contain a 'symbols' or 'additive-symbols' descriptor,
 	or else the ''@counter-style'' rule is invalid.
 
+	If the specified <<counter-style-name>> is an [=ASCII case-insensitive=] match
+	for ''disc'', ''circle'', ''square'', ''disclosure-open'', or ''disclosure-closed''
+	(any of the predefined symbolic counter styles),
+	using ''extend'' extends from the “standard” definition of the rules
+	provided in the normative stylesheet
+	(rather than the exception allowing them to be drawn in a different, user-agent-specific fashion.)
+
 	If the specified counter style name isn't the name of any defined counter style,
 	it must be treated as if it was extending the ''decimal'' counter style.
 	If one or more ''@counter-style'' rules form a cycle with their ''extends'' values,
@@ -1462,9 +1471,6 @@ Numeric: ''decimal'', ''decimal-leading-zero'', ''arabic-indic'', ''armenian'',
 			(e.g., ༡, ༢, ༣, ..., ༩༨, ༩༩, ༡༠༠).
 	</dl>
 
-	The ''decimal'' counter-style must not be overridable with a ''@counter-style'' rule,
-	so that it is always available as the ultimate fallback style.
-
 	The following stylesheet fragment provides the normative definition of these predefined counter styles:
 
 	<pre class='stylesheet'><bdo dir=ltr>
@@ -1765,9 +1771,6 @@ Symbolic: ''disc'', ''circle'', ''square'', ''disclosure-open'', ''disclosure-cl
 			such as the HTML <{details}> element.
 	</dl>
 
-	The ''disc'' counter-style must not be overridable with a ''@counter-style'' rule,
-	so that the initial value of 'list-style-type' is well-known to the user agent.
-
 	The following stylesheet fragment provides the normative definition of these predefined counter styles:
 
 	<pre class='stylesheet'>