chromium/chrome/browser/resources/webui_gallery/demos/cr_tree/cr_tree_demo.html

<style>
  cr-tree:not([has-children]) ~ #clearAll,
  cr-tree:not([has-children]) ~ #collapseAll,
  cr-tree:not([has-children]) ~ #expandAll {
    display: none;
  }

  cr-tree[has-children] ~ #populateTree {
    display: none;
  }
</style>

<h1>cr-tree</h1>
<div class="demos">
  <cr-tree></cr-tree>

  <label id="iconVisibilityContainer">
    <input type="checkbox" id="iconVisibility" checked>
    Show folder icons
  </label>

  <button id="expandAll">Expand All</button>
  <button id="collapseAll">Collapse All</button>
  <button id="addItem">Add item</button>
  <button id="removeItem">Remove item</button>

  <div id="log"></div>
</div>