プラス、＋、追加のアイコン

<span class="dli-plus"></span>

.dli-plus {
  display: inline-block;
  vertical-align: middle;
  color: #333;
  line-height: 1;
  width: 1em;
  height: 0.1em;
  background: currentColor;
  border-radius: 0.1em;
  position: relative;
}

.dli-plus::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: inherit;
  border-radius: inherit;
  transform: rotate(90deg);
}