<style type="text/css">
  .box { width: 200px; height: 200px;}
  #brown { background-color: brown; margin-bottom: 99px; padding-bottom: 1px}
  #green { background-color: green; margin-top: 100px}
</style>
<div id="brown" class="box"/><div id="green" class="box"/>


Valid XHTML 1.0!