jquery.responsiveiframes

Make all your Iframes responsive, no hassle.

Add <script type="text/javascript" src="jquery.responsiveiframes-min.js"></script> to your code, and all your <iframe>s will become responsive.
Yes, this includes Youtube embeds.

If there's a specific Iframe that you'd like to leave untouched - you can give it the class "no-aspect-ratio".