SOURCE

<div class="flashing"> </div>

.flashing {
    position: relative;
    width: 360px;
    height: 360px;
    border-radius: 6px;
    background: #ffffff;
    margin: 100px auto;
    animation: breath 2s linear 0.8s infinite alternate;
}



@keyframes breath {
    0% {
        box-shadow: 0 2px 8px 0 rgba(yellow, 0.7);
    }
    100% {
        box-shadow: 0 2px 8px 0 rgba(yellow, 0.2);
    }
}