html,body {
    width:100vw;
    height: 100vh;
    margin: 0;
}
#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 {
        display: none;
    }
    body {
        background: url('poster.jpg');
        background-size: cover;
    }
}

#box {
    background-image: url(CO_white.png);
    background-repeat: no-repeat;
    position:absolute;
    width: 600px;
    height: 300px;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.parent-div {
text-align: center;
}