<!DOCTYPE html>
<html>
<head>
<style>
#container {
font: 100px/1 Ahem, sans-serif;
width: 200px;
height: 200px;
color: green;
}
#left-border-box {
shape-outside: 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>