<!DOCTYPE html>
<html lang="en">
<head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
    <title></title>
    <style>
    #mask {
      clip-path: circle(50%);
      width: 100px;
      height: 100px;
      opacity: 0.5;
    }

    #content {
      width: 100px;
      height: 100px;
      background-color: green;
    }
  </style>
</head>

<body>
    <div id="mask">
        <div id="content"></div>
    </div>
</body>
</html>