<html>
  <head>
    <style>
        #bottom {
            float: left;
            width: 520px;
            height: 356px;
            background-color:red;
        }

        #top {
            float:left;
            width: 520px;
            height: 356px;
            margin-top:-356px;
            overflow:hidden;
            background-color:green;
        }
    </style>
  
  </head>

<body>
    There should be no red visible below.
    <div style="width:520px">
        <div id="bottom"></div>
        <div id="top"></div>
    </div>

</body>
</html>