SOURCE

<div></div>

div {
    width: 200px;
    height: 300px;
    background: radial-gradient(20px at 0% 0%, transparent 20px, #4169E1 20px) 0% 0%,
        radial-gradient(20px at 100% 0%, transparent 20px, #4169E1 20px) 100% 0%,
        radial-gradient(20px at 0% 100%, transparent 20px, #4169E1 20px) 0% 100%,
        radial-gradient(20px at 100% 100%, transparent 20px, #4169E1 20px) 100% 100%;
    background-size: 50% 50%;
    background-repeat: no-repeat
}