div {
height: 300px;
background: red;
position: relative;
}
div:before {
content: '';
position: absolute;
top: 0; right: 0;
border-top: 80px solid white;
border-left: 80px solid red;
width: 0;
}
div:after{
content: '';
position: absolute;
left: 0; bottom: 0;
border-bottom: 80px solid white;
border-right: 80px solid red;
width: 0;
}