.video {
    position: relative;
    padding-bottom: 56.25%; /*пропорции видео 16:9 */
    padding-top: 25px;
    height: 0;
    margin-bottom: 20px;
}

.video iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}