add_filter( 'embed_oembed_html', function ( $html, $url, $attr ) {
	if ((stripos($html,'<iframe') !== false || stripos($html,'<object') !== false || stripos($html,'<embed') !== false) && stripos($html,'flex-video') === false ) {
        $html = str_replace("<iframe","<iframe class='embed-responsive-item'",$html);
        return '<div class="embed-responsive embed-responsive-16by9">'.$html.'</div>';
    } else {
        return $html;
    }
}, 9999, 3);