.expandable-tree {
  --marker-width: 0.7rem;
}

.expandable-tree ul {
  margin-left: var(--marker-width);
  list-style-type: none;
  padding-left: 0;
  margin-top: 0;
  margin-bottom: 0;
}
.expandable-tree > ul {
  margin-left: 0;
}

.expandable-tree details {
  margin: 0;
}
.expandable-tree summary {
  font-weight: bold;
  list-style-type: none;
  padding-left: 0;
}
.expandable-tree summary::before {
  display: inline-block;
  width: var(--marker-width);
  font-size: 0.8em;
  vertical-align: 10%;
}
.expandable-tree details[open] summary::before {
  content: "▼";
}
.expandable-tree details:not([open]) summary::before {
  content: "▶";
}