<!doctype html>
<meta charset=utf-8>
<style>
.hr {
  color: gray;
  border-style: inset;
  border-width: 1px;
  margin: 0.5em auto;
}

.green {
  color: green;
}

.no-inset {
  border-style: solid;
}
</style>
<div class='hr'></div>
<div class='hr no-inset'></div>
<div class='hr no-inset'></div>
<div class='hr green no-inset'></div>