<div class="item"></div>

.item {
    background: black;
    height: 50px;
    width: 50px;
    animation: fade 3s linear infinite;
}

@keyframes fade {
    0% {
        opacity: 1;
    }
    25% { 
        opacity: 0.6;
    }
    50% {
        opacity: 0;
    }
    75% { 
        opacity: 0.7;
    }
    100% {
        opacity: 1;
    }
}