Bu sekil:

div {
height: 80px;
background: red;
position: relative;
}

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

Ek Olarak: Burda olusturdum outputu görmeniz icin:

https://jsfiddle.net/f7jttfgm/