[css-nesting-1] Specificity of '&' is zero without a parent rule (w3c#11135)

Resolves w3c#10196.

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>

Author: andruud

css-nesting-1/Overview.bs

@@ -966,7 +966,8 @@ Nesting Selector: the ''&'' selector {#nest-selector}
 	The <a>specificity</a> of the <a>nesting selector</a>
 	is equal to the largest specificity among the complex selectors
 	in the parent style rule's selector list
-	(identical to the behavior of '':is()'').
+	(identical to the behavior of '':is()''),
+	or zero if no such selector list exists.
 
 	<div class="example">
 		For example, given the following style rules: