#videoBG {
    position:fixed;
    z-index: -1;
}
@media (min-aspect-ratio: 16/9) {
    #videoBG {
        width:100%;
        height: auto;
    }
}
@media (max-aspect-ratio: 16/9) {
    #videoBG { 
        width:auto;
        height: 100%;
    }
}
@media (max-width: 767px) {
    #videoBG {
        width:auto;
        height: 100%;
    }
    body {
        background: url('poster.jpg');
        background-size: cover;
    }
}