@charset "UTF-8";

.youtube {
    width:100%;
position: relative;
padding-bottom: 75%; // これが縦横比
height: 0;
overflow: hidden;
margin-bottom:20px;
}
.youtube iframe {
position: absolute;
top: 0;
left: 0;
width: 100% !important;
height: 100% !important;
}