Template:Tree list/sandbox/styles.css

.treeview li {
  display: block;
  position: relative;
  margin: 0;
  padding-left: 0.8rem;
  list-style-type: none;
  list-style-image: none;
  
}

.treeview ul {
  margin: 0;
  padding-left: 0;
}

.treeview ul li {
  border-left: 2px solid #ccc;
}

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

.treeview ul li::before {
  content: '';
  display: block;
  position: absolute;
  top: -0.25rem;
  left: -2px;
  width: 0.7rem;
  height: 0.8rem;
  border: solid #ccc;
  border-width: 0 0 2px 2px;
}

.treeview > ul >li::before {
  border-color: transparent;
}

.treeview > ul >li {
  border-color: transparent;
}