A type of list that may contain sub-level nested groups that can be collapsed and expanded.
<ul role="tree">
<li role="treeitem">
Part 1
<ul>
<li role="treeitem">Chapter 1</li>
<li role="treeitem">Chapter 2</li>
<li role="treeitem">Chapter 3</li>
</ul>
</li>
<li role="treeitem">
Part 2
<ul>
<li role="treeitem">Chapter 4</li>
<li role="treeitem">Chapter 5</li>
<li role="treeitem">Chapter 6</li>
</ul>
</li>
<li role="treeitem">
Part 3
<ul>
<li role="treeitem">Chapter 7</li>
<li role="treeitem">Chapter 8</li>
<li role="treeitem">Chapter 9</li>
</ul>
</li>
</ul>