[cssom] Allow CSSImportRule.styleSheet to be null.

Fixes w3c#8608

Author: emilio

cssom-1/Overview.bs

@@ -2044,7 +2044,7 @@ The <code>CSSImportRule</code> interface represents an <code>@import</code> at-r
 interface CSSImportRule : CSSRule {
   readonly attribute USVString href;
   [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
-  [SameObject] readonly attribute CSSStyleSheet styleSheet;
+  [SameObject] readonly attribute CSSStyleSheet? styleSheet;
   readonly attribute CSSOMString? layerName;
   readonly attribute CSSOMString? supportsText;
 };
@@ -2059,7 +2059,7 @@ attribute of the associated <a>CSS style sheet</a>.
 The <dfn attribute for=CSSImportRule>media</dfn> attribute must return the value of the {{StyleSheet/media}}
 attribute of the associated <a>CSS style sheet</a>.
 
-The <dfn attribute for=CSSImportRule>styleSheet</dfn> attribute must return the associated <a>CSS style sheet</a>.
+The <dfn attribute for=CSSImportRule>styleSheet</dfn> attribute must return the associated <a>CSS style sheet</a>, if any, or null otherwise.
 
 The <dfn attribute for=CSSImportRule>layerName</dfn> attribute must return the [=layer name=] declared in the at-rule itself,
 or an empty string if the layer is anonymous,
@@ -2068,7 +2068,7 @@ or null if the at-rule does not declare a layer.
 The <dfn attribute for=CSSImportRule>supportsText</dfn> attribute must return the <<supports-condition>> declared in the at-rule itself,
 or null if the at-rule does not declare a supports condition.
 
-Note: An <code>@import</code> at-rule always has an associated <a>CSS style sheet</a>.
+Note: An <code>@import</code> at-rule might not have an associated <a>CSS style sheet</a> (e.g., if it has a non-matching <code>supports()</code> condition).
 
 
 ### The {{CSSGroupingRule}} Interface ### {#the-cssgroupingrule-interface}