Remove dimensions from oEmbed videos so that YouTube and other oEmbedded videos resize to fit device width. This only works for videos that use <a href="http://codex.wordpress.org/Embeds">oEmbed</a>.

functions.php

// VIDEO
// remove dimensions from oEmbed videos
add_filter( 'embed_oembed_html', 'tdd_oembed_filter', 10, 4 ) ; 
function tdd_oembed_filter($html, $url, $attr, $post_ID) {
    $return = '<figure class="video-container">'.$html.'</figure>';
    return $return;
}