.img { text-align:center; background-color:#9999FF;} /* height:800px; ou autre... */
img { max-width: 100%; }
@media (min-device-width:600px) {
    img[data-src-600px] {
        content: attr(data-src-600px, url);
    }
}
@media (min-device-width:800px) {
    img[data-src-800px] {
        content: attr(data-src-800px, url);
    }
}

.video-container {
position: relative;
padding-bottom: 56.25%;
height: 0;
overflow: hidden;
max-width:100%;
}
.video-container iframe,  
.video-container object,  
.video-container embed {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}