<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#positive, #negative {
background-color: green;
height: 20px;
}
#margin {
height: 30px;
</style>
</head>
<body>
<div id="positive"></div>
<div id="margin"></div>
<div id="negative"></div>
</body>
</html>