thin_border

last update: 2016/07/27
<!doctype html>
<title>title</title>
<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1, maximum-scale=1">

<style>
div {
  margin: 20px 10px;
  padding-top: 6px;
}
.normal {
  height: 100px;
  border-top: #333 1px solid;
}
.thin {
  height: 100px;
  position: relative;
}
.thin::after {
  content: "";
  width: 100%;
  height: 1px;
  border-top: #333 1px solid;
  position: absolute;
  top: 0;
  left: 0;
  -webkit-transform: scaleY(0.5);
  transform: scaleY(0.5);
}
</style>

<div class="normal">normal 1px</div>
<div class="thin">thin scaleY(0.5)</div>
sample