automatically adapts the width of an iframe to the available width, needs ratio set via width/height attributes and width set to 100% via CSS

responsive-iframe.js

/* Responsive iFrames
- - - - - - - - - - - - - - - - - - - -  */
var $iframes = $('.iframe-responsive');
if($iframes.length) {
  var adaptIframes = function() {
    $iframes.each(function(index, item) {
      var $iframe = $(item);
      var ratio = $iframes.attr('width') / $iframes.width();
      $iframe.css('height', $iframes.attr('height') / ratio);
    });
  };

  adaptIframes();
  $(window).on('resize', adaptIframes);
}