はてなマーク、クエスチョンマーク（丸囲み）のアイコン

<span class="dli-question-circle"><span></span></span>

.dli-question-circle {
  display: inline-block;
  vertical-align: middle;
  color: #333;
  line-height: 1;
  position: relative;
  width: 1em;
  height: 1em;
  border: 0.1em solid currentColor;
  border-radius: 50%;
  box-sizing: content-box;
}

.dli-question-circle > span {
  position: relative;
  width: 0.1em;
  height: 0.1504em;
  background: currentColor;
  border-radius: 0.05em 0 0.025em 0.025em;
  position: absolute;
  top: 9%;
  left: 0;
  right: 0;
  bottom: 0;
  margin: auto;
}

.dli-question-circle > span::before {
  content: '';
  position: absolute;
  left: 0.05em;
  bottom: calc(100% - 0.1em);
  width: 0.168em;
  height: 0.168em;
  border: 0.1em solid currentColor;
  border-left-color: transparent;
  border-radius: 50%;
  box-sizing: content-box;
  transform: translateX(-50%) rotate(-45deg);
}

.dli-question-circle > span::after {
  content: '';
  position: absolute;
  top: calc(100% + 0.06667em);
  left: 0;
  width: inherit;
  height: 0.1em;
  background: currentColor;
  border-radius: 50%;
}