<!DOCTYPE html>
<html>
<head>
<style>
#container {
    font: 100px/1 Ahem, sans-serif;
    -webkit-font-smoothing: antialiased;
    width: 200px;
    height: 200px;
    color: green;
}

#left-border-box {
    float: left;
    border-radius: 50px;
    width: 100px;
    height: 100px;
    background-color: blue;
    background-clip: border-box;
}
</style>
</head>
<body>
    <div id="container">
        <div id="left-border-box"></div>
        X
    </div>
</body>
</html>