* {
    margin: 0;
    padding: 0;
}

#box {
    width: 200px;
    height: 200px;
    background: #00f;
    animation: run 2s linear 1s infinite normal forwards running;
}

@keyframes run {
    form {
        width: 200px;
    }
    10% {
        width: 100px;
    }
    40% {
        width: 400px;
    }
    80% {
        width: 800px;
    }
    to {
        width: 600px;
    }
}