<!DOCTYPE html>
<html>
<head>
  <style type="text/css">
    .box {
      width: 100px;
      height: 100px;
      margin: 10px;
      background-color: red;
    }
  </style>
</head>
<body>
  
  <!-- You should see no red -->
  <div class="box" style="box-shadow: 150px 150px 0 green inset"></div>
  <div class="box" style="box-shadow: -150px -150px 0 green inset"></div>

</body>
</html>