<!doctype html>
<html><head>
<title>Circular border</title>
<style>
body { margin: 0 }
div {
margin-left: 8px; margin-top: 8px;
width: 50px; height: 50px;
border: 10px solid black;
border-radius: 10px;
}
div > div {
margin: 0; width: 50px; height: 50px;
border-radius: 0;
background: black;
border: none;
}
</style>
</head>
<body><div><div></div></div></body></html>