<!DOCTYPE html>
<html>
<head>
<style type="text/css">
html, body {
 margin: 0; padding: 5px;
}
div {
 height: 20px;
 background-color: green;
}
#big {
 background-color: blue;
 margin-bottom: 20px;
}
</style>
</head>
<body>
<div id="big"></div>
<div></div>
</body>
</html>