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