<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>a {
  display: inline-block;
  text-decoration: none;
  color: #FFF;
  font-weight: bold;
  text-align: center;
  margin: 10px;
  width: 100px;
  padding: 0.5em 2em;
}
span{
  color: #FF3399;
}
.btn{
  position: relative;
  background: #000;
}
.btn:before {
  content: '';
  position: absolute;
  z-index: -2;
  height: 10px;
  box-sizing: border-box;
  width: 100%;
  top: 100%;
   top: calc(100% - 2px);
  left: 0;
  background: linear-gradient(-90deg, #000 1px, transparent 0) 0 1px,
              linear-gradient(90deg, #000 4px, #fff 0) 0 1px;
  background-color: #142c5b;
  background-position: left bottom;
  background-repeat: repeat-x;
  background-size: 10px 10px;
  transition: .2s;
}</style>
</head>
<body>
<a class="btn" href="#">お し て <span>&#9829;</span>
</a>
<script>
</script>
</body>
</html>
a {
  display: inline-block;
  text-decoration: none;
  color: #FFF;
  font-weight: bold;
  text-align: center;
  margin: 10px;
  width: 100px;
  padding: 0.5em 2em;
}
span{
  color: #FF3399;
}
.btn{
  position: relative;
  background: #000;
}
.btn:before {
  content: '';
  position: absolute;
  z-index: -2;
  height: 10px;
  box-sizing: border-box;
  width: 100%;
  top: 100%;
   top: calc(100% - 2px);
  left: 0;
  background: linear-gradient(-90deg, #000 1px, transparent 0) 0 1px,
              linear-gradient(90deg, #000 4px, #fff 0) 0 1px;
  background-color: #142c5b;
  background-position: left bottom;
  background-repeat: repeat-x;
  background-size: 10px 10px;
  transition: .2s;
}