<!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>