1像素

• <div class="border3">
    <div class="content">伪类设置的边框</div>
  </div>

• .border3{
    position: relative;
  }
  
  .border3:after{
    content: '';
    display:block;
    position: absolute;
    left:0;
    top:0;
    width: 200%;
    height: 200%;
    border: 1px solid red;
    -webkit-transform-origin: 0 0;
    -moz-transform-origin: 0 0;
    -ms-transform-origin: 0 0;
    -o-transform-origin: 0 0;
    transform-origin: 0 0;
    -webkit-transform: scale(0.5, 0.5);
    -ms-transform: scale(0.5, 0.5);
    -o-transform: scale(0.5, 0.5);
    transform: scale(0.5, 0.5);
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
  }