<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<style>
body { margin: 0px; }
.outer {
        margin: -100px;
	width: 400px;
        height: 500px;
	background-color: green;
}
.outer:hover {
        background-color: red;
}
.inner {
	visibility: hidden;
	width: 100%;
	height: 50%;
	background-color: cyan;
}
</style>
</head>

<body>
<div class="outer" 
     onmouseover="firstChild.style.visibility='visible'" 
     onmouseout="firstChild.style.visibility='hidden'"><div class="inner"></div></div>
</body>
</html>