.successmessage {
    margin-top: 20px;
}

.iframe-container {
    position: relative;
    overflow: hidden;
    width: 100%;
    padding-top: 33%; /* 16:9 aspect ratio */
}
.iframe-container iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100% !important;
    height: 100%;
    border: 0;
}