SOURCE

<div class='corner'></div>

.corner{
  width:200px;
  height:200px;
  background:linear-gradient(135deg,transparent 15px ,yellowgreen 0) top left,
             linear-gradient(-135deg,transparent 15px,yellowgreen 0) top right,
             linear-gradient(45deg,transparent 15px,yellowgreen 0) bottom left,
             linear-gradient(-45deg,transparent 15px,yellowgreen 0) bottom right;
  background-size:50% 50%;
  background-repeat:no-repeat;
  margin:50px;
}