.banner_div {
    margin: 10px auto 10px; 
    max-width: 728px; 
    max-height: 90px;
    width: 100%;
    height: 100%;
}
.banner_div > iframe {
    padding: 0px; 
    margin: 0px;
    width: 100%;
    height: 100%;
    max-height: 90px;
}

@media only screen and (max-width: 727px) {

    .banner_div {
        padding-top: 12.362637362637%;
        position: relative;        
    }    

    .banner_div > iframe {
        position: absolute;
        top: 0;
        left: 0;    
        
        height: 0;
        max-height: 100%;
        max-width: 100%;
        min-height: 100%;
        min-width: 100%;
        width: 0;        
    }
}

@media only screen and (min-width: 728px) {
}