<!doctype html>
<html><head>
<title>Curved stripe border (HTML)</title>
<style>
body { margin: 0 }
div {
box-sizing: border-box;
width: 180px;
height: 180px;
border: 30px double black;
border-radius: 60px / 40px;
margin: 20px;
}
</style>
</head><body>
<div></div>
</body></html>