 #box3 -->
</div> <!-- #box3_parent -->

<style type="text/css" media="screen">
  /* When an element itself has display: none, we can still figure out what
     its dimensions will be _when_ it is made visible. */

  #box3_parent {
    width: 400px;
  }
  
  #box3 {
    display: none;
    padding: 10px;
    margin: 5px;
    border: 3px solid #000;
  }
</style>

<!-- Deeply hidden, statically-positioned element with width constrained by ancestor. -->
<div id="box4_ancestor">
  <div id="box4_parent">
    <div