別窓のアイコン

<span class="dli-external-link"><span></span></span>

.dli-external-link {
  display: inline-block;
  vertical-align: middle;
  color: #333;
  line-height: 1;
  width: 0.9em;
  height: 0.9em;
  border: 0.1em solid currentColor;
  border-radius: 0.1em;
  background: #fff;
  box-sizing: content-box;
  position: relative;
}

.dli-external-link > span {
  position: absolute;
  top: -0.2em;
  right: -0.2em;
  width: 45%;
  height: 45%;
  border: 0.1em solid currentColor;
  border-bottom: 0;
  border-left: 0;
  background: #fff;
  box-shadow: -0.1em 0.1em 0 0.1em #fff;
  box-sizing: border-box;
}

.dli-external-link > span::before {
  content: '';
  position: absolute;
  top: -0.05em;
  right: -0.1em;
  width: 0.1em;
  height: 0.9em;
  background: currentColor;
  transform: rotate(45deg);
  transform-origin: top center;
}