html {
    box-sizing: border-box;
}
*, *:before, *:after {
    box-sizing: inherit;
}
body{
    height: 100vh;
    width: auto;
    background-image: url(../img/intro_mobile_3.jpg);
    background-repeat: no-repeat;
    background-size: cover;
    background-position: top left;
    background-attachment: fixed;
}
@media (min-width: 768px) {
    body{
        background-image: url(../img/intro_desktop.jpg);
        background-position: center center;
    }
}