Responsive WordPress oEmbeds

functions.php

<?php

function responsive_oembed_filter($markup, $url, $attr, $post_ID) {
    return '<div class="video-container">' . $markup . '</div>';
}

add_filter( 'embed_oembed_html', 'responsive_oembed_filter');

style.css

.video-container {
    position: relative;
    padding-bottom: 56.25%;
    padding-top: 5px;
    height: 0;
}

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