SOURCE

<div class="container"></div>
<div class="container2"></div>

.container {
    width: 200px;
    height: 100px;
    background: #58a;
    background: linear-gradient(135deg, transparent 15px, #58a 0) top left, linear-gradient(-135deg, transparent 15px, #58a 0) top right, linear-gradient(-45deg, transparent 15px, #58a 0) bottom right, linear-gradient(45deg, transparent 15px, #58a 0) bottom left;
    background-size: 50% 50%;
    background-repeat: no-repeat;
}

.container2 {
    margin-top: 50px;
    width: 200px;
    height: 100px;
    background: #58a;
    background: radial-gradient(circle at top left, transparent 15px, #58a 0) top left, radial-gradient(circle at top right, transparent 15px, #58a 0) top right, radial-gradient(circle at bottom right, transparent 15px, #58a 0) bottom right, radial-gradient(circle at bottom left, transparent 15px, #58a 0) bottom left;
    background-size: 50% 50%;
    background-repeat: no-repeat;
}