缩放实现0.5px

一、

&::after {
  content: "";
  position: absolute;
  top: -50%;
  bottom: -50%;
  left: -50%;
  right: -50%;
  border: 1px solid #e9e9e9;
  border-radius: 2px;
  -webkit-transform: scale(0.5);
  transform: scale(0.5);
}

二、

&::after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 200%;
  height: 200%;
  transform-origin: 0 0;
  border-width: 1px;
  border-style: solid;
  border-radius: 50%;
  -webkit-transform: scale(0.5);
  transform: scale(0.5);
  box-sizing: border-box;
}