SOURCE

<span class='btn'>而我哦</span>

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font-size偶数<br />
http://imweb.io/topic/5848d0fc9be501ba17b10a94

.btn{
  position: relative;
  display: inline-block;
  padding: 7px 10px;
  box-sizing: content-box;
  border: none;
  line-height: 1;
  color: #333;
  background: none;
  font-size: 14px;
}
.btn:after{
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  border: 1px solid #ccc;
  border-radius: 8px;
  box-sizing: border-box;
  width: 200%;
  height: 200%;
  transform: scale(0.5);
  transform-origin: left top;
}