css3 media

    @media screen and (max-width: 320px) {
        .cloud{position: fixed;left: 0;top: 70%;width: 150px;}
        .cloud2{position: fixed;right: 0;top: 30%;width: 50px;}
        .cloud3{position: fixed;left: 0;top: 23%;width: 100px;}
    }
    @media screen and (min-width: 321px) and (max-width: 375px) {
        .cloud{position: fixed;left: 0;top: 72%;width: 150px;}
        .cloud2{position: fixed;right: 0;top: 32%;width: 50px;}
        .cloud3{position: fixed;left: 0;top: 25%;width: 100px;}
    }
    @media screen and (min-width: 376px) {
        .cloud{position: fixed;left: 0;top: 68%;width: 150px;}
        .cloud2{position: fixed;right: 0;top: 32%;width: 50px;}
        .cloud3{position: fixed;left: 0;top: 25%;width: 100px;}
    }
    @media screen and (max-height: 480px) {
        .cloud{position: fixed;left: 0;top: 81%;width: 150px;}
        .cloud2{position: fixed;right: 0;top: 35%;width: 50px;}
        .cloud3{position: fixed;left: 0;top: 27%;width: 100px;}
    }