[css-counter-styles] Further clarify that undefined counter styles don't compute to decimal, just act like decimal. Fixes w3c#1616.

Author: tabatkins

css-counter-styles-3/Overview.bs

@@ -232,7 +232,7 @@ Counter algorithms: the 'system' descriptor</h3>
 	the counter's representation based on the counter value.  For example,
 	''cyclic'' counter styles just cycle through their symbols repeatedly,
 	while ''system/numeric'' counter styles interpret their symbols as digits and
-	build their representation accordingly.  
+	build their representation accordingly.
 	The systems are defined in the following subsections.
 
 <!--
@@ -1035,31 +1035,31 @@ Marker characters: the 'symbols' and 'additive-symbols' descriptors</h3>
 	The 'symbols' and 'additive-symbols' descriptors specify the symbols
 	used by the marker-construction algorithm specified by the 'system' descriptor.
 	The ''@counter-style'' rule must have a valid 'symbols' descriptor
-	if the counter system is 
-	''cyclic'', ''system/numeric'', ''system/alphabetic'', ''symbolic'', or ''system/fixed'', 
+	if the counter system is
+	''cyclic'', ''system/numeric'', ''system/alphabetic'', ''symbolic'', or ''system/fixed'',
 	or a valid 'additive-symbols' descriptor
-	if the counter system is ''additive''; 
+	if the counter system is ''additive'';
 	otherwise, the ''@counter-style'' is invalid and must be ignored.
 
 	Some counter systems specify that the 'symbols' descriptor must have at least two entries.
-	If the counter style's system is such, 
-	and the 'symbols' descriptor has only a single entry, 
+	If the counter style's system is such,
+	and the 'symbols' descriptor has only a single entry,
 	the descriptor is invalid and must be ignored.
 
 	Each entry in the 'symbols' descriptor's value defines a <dfn export>counter symbol</dfn>,
-	which is interpreted differently based on the counter style's system.  
+	which is interpreted differently based on the counter style's system.
 	Each entry in the 'additive-symbols' descriptor's value defines an <dfn export>additive tuple</dfn>,
 	which consists of a <a>counter symbol</a> and a non-negative integer weight.
-	Each weight must be a non-negative integer, 
-	and the <a>additive tuples</a> must be specified in order of strictly descending weight; 
+	Each weight must be a non-negative integer,
+	and the <a>additive tuples</a> must be specified in order of strictly descending weight;
 	otherwise, the declaration is invalid and must be ignored.
 
 	<a>Counter symbols</a> may be strings, images, or identifiers,
 	and the three types can be mixed in a single descriptor.
 	Counter representations are constructed by concatenating counter symbols together.
 	Identifiers are rendered as strings containing the same characters.
 	Images are rendered as inline replaced elements.
-	The <a href="https://drafts.csswg.org/css3-images/#default-object-size">default object size</a> of an image <a>counter symbol</a> 
+	The <a href="https://drafts.csswg.org/css3-images/#default-object-size">default object size</a> of an image <a>counter symbol</a>
 	is a 1em by 1em square.
 
 	Note: If using identifiers rather than strings to define the symbols,
@@ -1275,7 +1275,8 @@ Extending 'list-style-type', ''counter()'', and ''counters()''</h2>
 	</pre>
 
 	If a <<counter-style-name>> is used that does not refer to any existing counter style,
-	it must act identically to the ''decimal'' counter style.
+	it must act identically to the ''decimal'' counter style
+	(but does not <em>[=computed value|compute=]</em> to ''decimal'').
 
 
 <!--
@@ -2468,7 +2469,7 @@ Additional Predefined Counter Styles</h2>
 	The Internationalization Working Group maintains a large list of predefined ''@counter-style'' rules for various world languages
 	in their <a href="https://www.w3.org/TR/predefined-counter-styles/">Predefined Counter Styles</a> document.
 	[[predefined-counter-styles]]
-	
+
 	These additional counter styles are not intended to be supported by user-agents by default,
 	but can be used by users or authors copying them directly into style sheets.