wp-responsive-oembeds.php

/*
==== Wrap video embeds in a div so they can be made responsive

(will need to have appropriate css in place)

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

*/

add_filter( 'embed_oembed_html', 'fq_wrap_embed', 10, 3 );

function fq_wrap_embed( $html, $url, $attr ) {
        return '<div class="embed-container">' . $html . '</div>';
}