<!DOCTYPE html>
<html>
<head>
<style>
body {
    margin: 0;
    padding: 0;
}

#green-square {
    background-color: green;
    width: 100px;
    height: 100px;
}
</style>
</head>
<body>
<div id="green-square"></div>
</body>
</html>
