.embed-wrapper { /* in order for the video to still have a smaller width than 100% of the container, a second wrapper div is needed to apply the max-width */
    margin: auto; /* to center it */
    max-width: 640px;
    width: 100%; /* needed to center element in older IEs */
}

.embed-container {
    padding-bottom: 56.25%; /* this is based on a 16:9 ratio, can be changed */
    position: relative;
    overflow: hidden;
    max-width: 100%;
    height: 0;
}

    .embed-container iframe {
        position: absolute;
        left: 0;
        top: 0;
        width: 100%;
        height: 100%;
    }