<!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>♥</span>
</a>
<script>
</script>
</body>
</html>