Pure CSS Circle Timer

HTML

<div class="timer">
    <div class="mask"></div>
</div>

CSS

.timer {
    background: -webkit-linear-gradient(left, skyBlue 50%, #eee 50%);
    /* Foreground color, Background colour */
    border-radius: 100%;
    height: 100px;
    /* Height and width */
    position: relative;
    width: 100px;
    /* Height and width */
    animation-name: time;
    animation-duration: 20s;
    animation-timing-function: linear;
    animation-iteration-count: infinite;
}
.mask {
    border-radius: 100% 0 0 100% / 50% 0 0 50%;
    height: 100%;
    left: 0;
    position: absolute;
    top: 0;
    width: 50%;
   
    animation-name: mask;
    animation-duration: 20s;
    animation-timing-function: linear;
    animation-iteration-count: infinite;
    /* Animation time and number of steps (halved) */
    -webkit-transform-origin: 100% 50%;
}
@-webkit-keyframes time {
    100% {
        -webkit-transform: rotate(360deg);
    }
}
@-webkit-keyframes mask {
    0% {
        background: #eee;
        /* Background colour */
        -webkit-transform: rotate(0deg);
    }
    50% {
        background: #eee;
        /* Background colour */
        -webkit-transform: rotate(-180deg);
    }
    50.01% {
        background: skyBlue;
        /* Foreground colour */
        -webkit-transform: rotate(0deg);
    }
    100% {
        background: skyBlue;
        /* Foreground colour */
        -webkit-transform: rotate(-180deg);
    }
}