<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <meta http-equiv="Content-type" content="text/html; charset=utf-8"/>
    <title>with_hover</title>
  </head>

  <body id="with_hover">
    <style type="text/css">
      .hidden_until_hover {display: none;}
      .wrapper:hover .hidden_until_hover { display: block }
    </style>
    <div class="wrapper">
      Some text here so the wrapper has size
      <div class="hidden_until_hover">Here I am</div>
    </div>
  </body>
</html>