[css-nesting-1] Align with Web IDL specification (w3c#5767)

Author: autokagami

css-nesting-1/Overview.bs

@@ -29,6 +29,7 @@ Module Interactions</h3>
 	This module introduces selectors that extend the [[SELECTORS4]] module.
 
 <pre class=idl>
+[Exposed=Window]
 interface CSSNestingRule : CSSRule {
   attribute CSSOMString selectorText;
   [SameObject] readonly attribute CSSRuleList cssRules;