html, body {
    margin: 0;
    width: 100%;
    min-height: 100%;
}

body {
    min-height: 100vh;

    background-image: url("images/forest.jpg");
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
    background-attachment: fixed;
}

#rat {
    position: fixed;

    width: 60px;

    bottom: 10px;
    left: -70px;

    z-index: 10;
}
#rat.running {
    animation: ratRun 8s linear forwards;
}

@keyframes ratRun {
    from {
        left: -70px;
    }

    to {
        left: 100vw;
    }
}