<!DOCTYPE html>
<style>
.container {
    width: 150px;
    height: 50px;
    font: 50px/1 Ahem;
    overflow: hidden;
    background-color: red;
    color: green;
}
.float {
    width: 100px;
    height: 100px;
    float: left;
    background-color: green;
    margin-top: -50px;
    shape-outside: inset(0);
}
</style>
<script src="../../../resources/ahem.js"></script>
<body>
    <p>You should see one green rectangle. You should not see any red.</p>
    <div class="container">
        <div class="float"></div>
        X
    </div>
</body>