[css-counter-styles-3] Explicitly tell when the counter style is not …

…defined (#11896)

@counter-style does not define a counter style (but is still a valid rule) when some descriptors are declared with a valid value that do not meet the requirement of the declared (or default) system.

css-counter-styles-3/Overview.bs

@@ -272,7 +272,8 @@ Cycling Symbols: the ''cyclic'' system</h4>
 	the second <a>counter symbol</a> (if it exists) is used as the representation of the value 2, etc.
 
 	If the system is ''cyclic'',
-	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>.
+	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is defined over all counter values.
 
 	<div class=example>
@@ -321,7 +322,8 @@ Exhaustible Symbols: the ''system/fixed'' system</h4>
 	such as circled digits.
 
 	If the system is ''system/fixed'',
-	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>.
+	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is defined over counter values in a finite range,
 	starting with the <a>first symbol value</a> and having a length equal to the length of the list of <a>counter symbols</a>.
 
@@ -381,7 +383,8 @@ Repeating Symbols: the ''symbolic'' system</h4>
 	and is also sometimes used for alphabetic-style lists for a slightly different presentation than what the ''system/alphabetic'' system presents.
 
 	If the system is ''symbolic'',
-	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>.
+	the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is defined only over strictly positive counter values.
 
 	<div class=example>
@@ -469,7 +472,8 @@ Bijective Numerals: the ''alphabetic'' system</h4>
 	and so on.
 
 	If the system is ''system/alphabetic'',
-	the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>.
+	the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is defined only over strictly positive counter values.
 
 	<div class=example>
@@ -541,7 +545,8 @@ Positional Numerals: the ''numeric'' system</h4>
 	and so on.
 
 	If the system is ''system/numeric'',
-	the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>.
+	the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is defined over all counter values.
 
 	<div class=example>
@@ -612,7 +617,8 @@ Accumulating Numerals: the ''additive'' system</h4>
 	and other numbering systems around the world.
 
 	If the system is ''additive'',
-	the '@counter-style/additive-symbols' descriptor must contain at least one <a>additive tuple</a>.
+	the '@counter-style/additive-symbols' descriptor must contain at least one <a>additive tuple</a>,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 	This system is nominally defined over all counter values
 	(see algorithm, below, for exact details).
 
@@ -709,7 +715,8 @@ Building from Existing Counter Styles: the ''extends'' system
 	rather than taking their initial values.
 
 	If a ''@counter-style'' uses the ''extends'' system,
-	it must not contain a '@counter-style/symbols' or '@counter-style/additive-symbols' descriptor.
+	it must not contain a '@counter-style/symbols' or '@counter-style/additive-symbols' descriptor,
+	otherwise the rule does not define a counter style (but is still a valid rule).
 
 	If the specified <<counter-style-name>> is an [=ASCII case-insensitive=] match
 	for ''disc'', ''circle'', ''square'', ''disclosure-open'', or ''disclosure-closed''
@@ -2615,7 +2622,7 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 			1. [=CSS/parse=] [=the given value=] as the descriptor associated with the attribute.
 
 			2. If the result is invalid according to the given descriptor's grammar,
-				or would cause the ''@counter-style'' rule to become invalid,
+				or would cause the ''@counter-style'' rule to not define a counter style,
 				do nothing and abort these steps.
 				(For example, some systems require the '@counter-style/symbols' descriptor to contain two values.)