[css-cascade] Export reset-only subproperty

Author: fantasai

css-cascade-3/Overview.bs

@@ -180,7 +180,7 @@ h1 {
 	that don't directly correspond to values of its <a>sub-properties</a>.
 	(In such cases, the <a>shorthand</a> will explicitly define the expansion of its values.)
 
-	In other cases, a property might be a <dfn>reset-only sub-property</dfn> of the shorthand:
+	In other cases, a property might be a <dfn export>reset-only sub-property</dfn> of the shorthand:
 	Like other <a>sub-properties</a>, it is reset to its initial value by the shorthand when unspecified,
 	but the shorthand might not include syntax to set the <a>sub-property</a>
 	to any of its other values.