SOURCE

<div class="chamfer" ></div>

.chamfer{
    background: #3b3; 
    background: linear-gradient(135deg, transparent 15px, #3b3 0) top left, 
			linear-gradient(-135deg, transparent 15px, #3b3 0) top right, 
			linear-gradient(-45deg, transparent 15px, #3b3 0) bottom right, 
			linear-gradient(45deg, transparent 15px, #3b3 0) bottom left; 	
    background-size: 50% 50%; 
    background-repeat: no-repeat;
}