/* apply a letter-wobble animation effect to cells */
.cell:nth-of-type(4n-6):not(.clicked) > div {
    animation: anim-rotate 15s ease 5s infinite;
}
@keyframes anim-rotate {
    0% {
        transform:rotate(0);
    }
    25% {
        transform:rotate(-10deg);
    }

    75% {
        transform:rotate(10deg);
    }
    100% {
        transform:rotate(0);
    }

}