@keyframes rotate-element {
    0%  {transform: rotate(0);}
    25% {transform: rotate(90);}
    50% {transform: rotate(180);}
    75% {transform: rotate(270);}
    100%{transform: rotate(360);}
}

#clock {
    animation-name: rotate-element;
    animation-duration: 10s;
    animation-iteration-count: infinite;
}