<ul class="tree">
  <li>
    <details open>
      <summary>Giant planets</summary>
      <ul>
        <li>
          <details>
            <summary>Gas giants</summary>
            <ul>
              <li>Jupiter</li>
              <li>Saturn</li>
            </ul>
          </details>
        </li>
        <li>
          <details>
            <summary>Ice giants</summary>
            <ul>
              <li>Uranus</li>
              <li>Neptune</li>
            </ul>
          </details>
        </li>
      </ul>
    </details>
  </li>
</ul>

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}

.tree li {
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}

.tree ul {
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}

.tree ul li {
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}

.tree ul li:last-child {
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}

.tree ul li::before {
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  top: calc(var(--spacing) / -2);
  left: -2px;
  width: calc(var(--spacing) + 2px);
  height: calc(var(--spacing) + 1px);
  border: solid #ddd;
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}

.tree summary {
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}

.tree summary::marker,
.tree summary::-webkit-details-marker {
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}

.tree summary:focus {
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}

.tree summary:focus-visible {
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.tree li::after,
.tree summary::before {
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  display: block;
  position: absolute;
  top: calc(var(--spacing) / 2 - var(--radius));
  left: calc(var(--spacing) - var(--radius) - 1px);
  width: calc(2 * var(--radius));
  height: calc(2 * var(--radius));
  border-radius: 50%;
  background: #ddd;
}

.tree summary::before {
  content: "+";
  z-index: 1;
  background: #696;
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}

.tree details[open] > summary::before {
  content: "−";
}