clarify authoring requirements for nested collections.

index.html

@@ -4785,10 +4785,12 @@ <h2>Definition of Roles</h2>
 			<rdef>listviewitem</rdef>
 			<div class="role-description">
 				<p>An item in a <rref>listview</rref>.</p>
-				<p>A <code>listviewitem</code> is focusable and can be selectable.</p>
+				<p>A <code>listviewitem</code> can be focusable and selectable.</p>
 				<p>
-					A <code>listviewitem</code> <a>element</a> can contain a sub-level group of elements that can be expanded or collapsed.
-					An expandable collection of <code>listviewitem</code> elements are enclosed in an element with the <rref>group</rref> <a>role</a>.
+					A <code>listviewitem</code> <a>element</a> can contain a nested collection of <code>listviewitem</code> elements.
+					A nested collection of <code>listviewitem</code> elements is enclosed in an element with the <rref>group</rref> <a>role</a>.
+					Authors may enable users to control the visibility and focusability of a nested collection, i.e., make it expandable and collapsable.
+					If the nested group of <code>listviewitem</code> elements can be expanded and collapsed, authors MUST specify a value for <sref>aria-expanded</sref> on the <code>listviewitem</code that contains the group.
 				</p>
 				<p>Authors MUST ensure [=elements=] with <a>role</a> <code>listviewitem</code> are <a>accessibility children</a> of an element with <a>role</a> <rref>listview</rref> or an element with <a>role</a> <rref>group</rref> that is the <a>accessibility child</a> of an element with role <rref>listviewitem</rref>.</p>
 				<p>