.modal-iframe-wrapper {
    position: relative;
    display: block;
    width: 100%;
    padding: 0;
    overflow: hidden;
}
.modal-iframe-wrapper:before {
    content: "";
    padding-top: 56.25%;
    display: block;
}

.modal-iframe-wrapper iframe {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: 0;
}