.responsive-iframe {
    position: relative;
    width: 100%;
    height: 0;
    padding-bottom: 75%; /* 4:3 Aspect Ratio */
    overflow: hidden;
}
.responsive-iframe iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: 0;
}