<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#sibling1 {
height: 20px;
background-color: green;
margin-bottom: 30px;
}
#sibling2 {
</style>
</head>
<body>
<div id="sibling1"></div>
<div id="sibling2"></div>
</body>
</html>