The parent collapses when I do not specify a height..
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<style type="text/css" >
#parent{width:500px;border:#000 thin groove}
#parent p{float:left}
</style>
</head>
<body>
<div id="parent">
<p height="100px" width="100px">text</p>
</div>
</body>
</html>
Add
overflow: hiddento your#parentdivExplanations as to why this works is at http://www.quirksmode.org/css/clearing.html