<!DOCTYPE html>
<html>
<body>

<div id="empty" style="position:fixed; z-index: 4; left:200px; top:200px; width: 0px">
  <div style="position:fixed; width:400px; height: 400px; background-color:green"></div>
</div>
<div style="width:400px; height: 400px; background-color:blue"></div>

</body>
</html>