<!DOCTYPE html>
<html>
<head>
  <style>
    div {
      border-width: 25px;
      border-style: solid;
      border-color: green;
    }
    .main {
      width: 100px;
      height: 100px;
    }
    .sub {
      width: 50px;
      height: 50px;
    }
    .sub-sub {
      width: 0px;
      height: 0px;
    }
    .low-opacity {
      opacity: 0.2;   
    }
  </style>
</head>
<body>
  <div class="main">
    <div class="sub low-opacity">
      <div class="sub-sub low-opacity"></div>
    </div>
  </div>
</body>
</html>