You can do it like the following:
html
<div class="topleft">
<div class="buttomright"></div>
</div>
and the css
div {
height: 300px;
background: silver;
position: relative;
}
.topleft:after {
content:'';
position: absolute;
top: 0;
left: 0;
border-top: 80px solid white;
border-right: 80px solid silver;
width: 0;
}
.buttomright:before {
content:'';
position: absolute;
bottom: 0;
right: 0;
border-bottom: 80px solid white;
border-left: 80px solid silver;
width: 0;
}
please see the JSFiddle