SOURCE

<input type="checkbox" id="awesome" />
<label for="awesome">
  Awesome!
</label>

input[type="checkbox"] + label::before {
  content: '\a0';
  /* 不换行空格 */
  display: inline-block;
  vertical-align: .2em;
  width: .8em;
  height: .8em;
  margin-right: .2em;
  border-radius: .2em;
  background: silver;
  text-indent: .15em;
  line-height: .65;
}

input[type="checkbox"] {
  position: absolute;
  clip: rect(0, 0, 0, 0);
}

input:checked + label::before {
  content: '\2713';
  background: yellowgreen;
}