Tree views in CSS

<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>

.tree{
  --spacing : 1.5rem;
  --radius  : 10px;
}

.tree li{
  display      : block;
  position     : relative;
  padding-left : calc(2 * var(--spacing) - var(--radius) - 2px);
}

.tree ul{
  margin-left  : calc(var(--radius) - var(--spacing));
  padding-left : 0;
}

.tree ul li{
  border-left : 2px solid #ddd;
}

.tree ul li:last-child{
  border-color : transparent;
}

.tree ul li::before{
  content      : '';
  display      : block;
  position     : absolute;
  top          : calc(var(--spacing) / -2);
  left         : -2px;
  width        : calc(var(--spacing) + 2px);
  height       : calc(var(--spacing) + 1px);
  border       : solid #ddd;
  border-width : 0 0 2px 2px;
}

.tree summary{
  display : block;
  cursor  : pointer;
}

.tree summary::marker,
.tree summary::-webkit-details-marker{
  display : none;
}

.tree summary:focus{
  outline : none;
}

.tree summary:focus-visible{
  outline : 1px dotted #000;
}

.tree li::after,
.tree summary::before{
  content       : '';
  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{
  z-index    : 1;
  background : #696 url('expand-collapse.svg') 0 0;
}

.tree details[open] > summary::before{
  background-position : calc(-2 * var(--radius)) 0;

Vertical lines

.tree ul li{
  border-left : 2px solid #ddd;
}

.tree ul li:last-child{
  border-color : transparent;
}

Horizontal lines

.tree ul li::before{
  content      : '';
  display      : block;
  position     : absolute;
  top          : calc(var(--spacing) / -2);
  left         : -2px;
  width        : calc(var(--spacing) + 2px);
  height       : calc(var(--spacing) + 1px);
  border       : solid #ddd;
  border-width : 0 0 2px 2px;
}

Remove the default styling from summaries

.tree summary{
  display : block;
  cursor  : pointer;
}

.tree summary::marker,
.tree summary::-webkit-details-marker{
  display : none;
}

.tree summary:focus{
  outline : none;
}

.tree summary:focus-visible{
  outline : 1px dotted #000;
}

Markers

.tree li::after,
.tree summary::before{
  content       : '';
  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;
}

Expand and collapse buttons

.tree summary::before{
  z-index    : 1;
  background : #696 url('expand-collapse.svg') 0 0;
}

.tree details[open] > summary::before{
  background-position : calc(-2 * var(--radius)) 0;
}