<!doctype html>
<meta charset="utf-8">
<title>Corner Clipped Background Color</title>
<style>
    html {
        background-color: green;
    }
    #a {
        top: 20px;
        left: 20px;
        position: absolute;
        width: 20px;
        height: 20px;
        background-color: black;
    }
</style>
<div id="a"></div>