<!DOCTYPE html>
<style>
    .clip {
        width: 40vw;
        height: 40vw;
        background-color: blue;
        -webkit-clip-path: polygon(10vw 10vw, 30vw 10vw, 30vw 30vw, 10vw 30vw);
    }
</style>
<body>
    <div class="clip"></div>
</body>