I am using Firefox3 on Ubuntu (And I found a bug in SO while at that :-D) The expected behavior is not to see any margin between the DIVs, while a margin is shown, originating from the P margins.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<style>
p{
background-color: transparent;
margin: 10px;
color: white;
}
div{
background-color: black;
margin:0;
width: 300px;
}
</style>
</head>
<body>
<div>
<p>aaaaaaaaaaa</p>
<p>bbbbbbbbbbb</p>
<p>ccccccccccc</p>
</div>
<div>
<p>aaaaaaaaaaa</p>
<p>bbbbbbbbbbb</p>
<p>ccccccccccc</p>
</div>
</body>
</html>