<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width">
<style>
body {
    margin: 0;
    padding: 0;
}

#expected {
    position: fixed;
    bottom: 0px;
    width: 256px;
    height: 256px;
    background-color: green;
}
</style>
</head>
<body>
<div id="expected"></div>
</body>
</html>