[cssom-1] Tweak CSSImportRule layerName attribute definition

Author: CanadaHonk

cssom-1/Overview.bs

@@ -2060,7 +2060,7 @@ 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>layerName</dfn> attribute may return the layer name declared in the at-rule itself,
+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,
 or null if the at-rule does not declare a layer.