<!DOCTYPE html>
<meta charset="utf-8">
<title>Filled Background with Rounded Corner</title>
<style>
    #a {
        width: 100px;
        height: 80px;
        border: 20px blue solid;
        border-top-right-radius: 20px;
        background-color: green;
        background-clip: border-box;
    }
    #shield {
        position: absolute;
        width: 30px;
        height: 30px;
        left: 120px;
        top: 5px;
        background-color: black;
    }
</style>
<body>
    <div id="a"></div>
    <!-- Hide the curved outside border to deal with imprecise rendering. -->
    <div id="shield"></div>
</body>