Simple HTML tree

Interactive demo

The Code

HTML

<details open>
  <summary>root</summary>
  <details>
  <summary>images</summary>
    <ul>
      <li>one</li>
      <li>two</li>
    </ul>
  </details>
</details>

The details can have an attribute name to group them.

CSS

details details {
    margin-left: 1rem
}

Très simple!