$border-poses: top, right, bottom, left;

@mixin border-retina ($border-color) {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 200%;
  height: 200%;
  transform: scale(.5);
  transform-origin: 0 0;
  pointer-events: none;
  box-sizing: border-box;
  @each $pos in $border-poses {
    border-#{$pos}: 1px solid $border-color;
  }
}

@mixin hairline ($border-color) {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 200%;
  height: 200%;
  transform: scale(.5);
  transform-origin: 0 0;
  pointer-events: none;
  box-sizing: border-box;
  border: 0 solid $border-color;
}