:root {
    background: url(http://tympanus.net/codrops-playground/assets/images/cssref/pseudo/root/sativa.png);
}

html {
    background: tomato;
}

:root:after {
    content: "I am generated!";
    color: white;
    display: block;
    width: 200px;
    height: 100px;
    line-height: 100px;
    position: fixed;
    top: 50%;
    left: 50%;
    margin-left: -100px;
    margin-top: -50px;
    background-color: purple;
    text-align: center;
    font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

body {
    background-color: #009966;
    color: #eee;
    font-size: 1.5em;
    padding: 1em;
    font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}