.iframe-16x9 {
    position: relative;
    padding-bottom: 56.25%;
    height: 0;
    overflow: hidden;
}
.iframe-4x3 {
    position: relative;
    padding-bottom: 75%;
    height: 0;
    overflow: hidden;
}
.iframe-16x9 iframe, .iframe-4x3 iframe {
    position: absolute;
    top:0;
    left: 0;
    width: 100%;
    height: 100%;
}

#gotop {
    display: none;
    position: fixed;
    right: 40px;
    bottom: 20px;    
    padding: 5px 10px 0 10px;    
    font-size: 20px;
    background: #ccc;
    opacity:0.7;
    border:1px solid #aaa;
    color: #888;
    border-radius:30px;
    cursor: pointer;
}