Untuk memotong kotak di CS

div {
    height: 300px;
    background: red;
    position: relative;
}

div:before {
    content: '';
    position: absolute;
    top: 1; right: 1;
    border-top: 80px solid white;
    border-left: 80px solid red;
    width: 0;
}
Prickly Peafowl