div.loading{
    position: absolute; left: 0; top: 0; right: 0; bottom:0; margin: auto;
    border: 5px solid #aaa; border-top-color: #444;
    width: 50px; height: 50px; border-radius: 25px;
    animation: rotation 600ms infinite linear;
}

@keyframes rotation {
    from {
        transform: rotate(0deg);
    }
    to {
        transform: rotate(360deg);
    }
}