<!DOCTYPE html>
<html lang="en">
<head>
<style>
div {
    width: 200px;
    height: 200px;
    background-color: green;
    border: 20px solid black;
    /* Even on missing reference box and overflowing context chose border-box. */
    -webkit-clip-path: circle(50% at 50% 50%);
}
</style>
</head>
<body>
<div><div>
</body>
</html>