javascript:
  var dom = {};
  dom.slides = document.querySelector('.reveal .slides');

  function getRemainingHeight(element, slideElement, height) {
    height = height || 0;
    if (element) {
      var newHeight, oldHeight = element.style.height;
      // Change the .stretch element height to 0 in order find the height of all
      // the other elements
      element.style.height = '0px';
      // In Overview mode, the parent (.slide) height is set of 700px.
      // Restore it temporarily to its natural height.
      slideElement.style.height = 'auto';
      newHeight = height - slideElement.offsetHeight;
      // Restore the old height, just in case
      element.style.height = oldHeight + 'px';
      // Clear the parent (.slide) height. .removeProperty works in IE9+
      slideElement.style.removeProperty('height');
      return newHeight;
    }
    return height;
  }

  function layoutSlideContents(width, height) {
    // Handle sizing of elements with the 'stretch' class
    toArray(dom.slides.querySelectorAll('section .stretch')).forEach(function (element) {
      // Determine how much vertical space we can use
      var limit = 5; // hard limit
      var parent = element.parentNode;
      while (parent.nodeName !== 'SECTION' && limit > 0) {
        parent = parent.parentNode;
        limit--;
      }
      if (limit === 0) {
        // unable to find parent, aborting!
        return;
      }
      var remainingHeight = getRemainingHeight(element, parent, height);
      // Consider the aspect ratio of media elements
      if (/(img|video)/gi.test(element.nodeName)) {
        var nw = element.naturalWidth || element.videoWidth, nh = element.naturalHeight || element.videoHeight;
        var es = Math.min(width / nw, remainingHeight / nh);
        element.style.width = (nw * es) + 'px';
        element.style.height = (nh * es) + 'px';
      } else {
        element.style.width = width + 'px';
        element.style.height = remainingHeight + 'px';
      }
    });
  }

  function toArray(o) {
    return Array.prototype.slice.call(o);
  }

  Reveal.addEventListener('slidechanged', function () {
    layoutSlideContents(#{attr 'revealjs_width', 960}, #{attr 'revealjs_height', 700})
  });
  Reveal.addEventListener('ready', function () {
    layoutSlideContents(#{attr 'revealjs_width', 960}, #{attr 'revealjs_height', 700})
  });
  Reveal.addEventListener('resize', function () {
    layoutSlideContents(#{attr 'revealjs_width', 960}, #{attr 'revealjs_height', 700})
  });