Ho trovato questo link molto utile per risolvere il problema del responsive sui video embed di youtube.
http://www.kuma.cloud/blog/incorporare-youtube-in-un-sito-con-responsiv…
<div class="rwd-video"> <iframe src="//www.youtube.com/embed/-lmlmt1Omew?rel=0" allowfullscreen="" frameborder="0" height="315" width="560"> </iframe> </div>
.rwd-video { height: 0; overflow: hidden; padding-bottom: 56.25%; padding-top: 30px; position: relative; } .rwd-video iframe, .rwd-video object, .rwd-video embed { height: 100%; left: 0; position: absolute; top: 0; width: 100%; }