iframe-transparent.js

window.addEventListener("load", function() {

  var self = window.self,
      parent = window.parent,
      setTransparentBackground = function(doc) {
          doc.getElementsByTagName('html')[0].style.background = 'transparent';
          doc.getElementsByTagName('body')[0].style.background = 'transparent';
      };

  // make current doc transparent
  setTransparentBackground(window.document);

  // Iterate upwards over all iframes frome here
  while(self != parent) {
      var parentIframes = parent.document.getElementsByTagName('iframe'),
          iframe = false,
          iframeDoc = false;

      // Find iframe holding this document in parent window
      for(var i=0; i < parentIframes.length; i++) {
          iframeDoc = parentIframes[i].contentDocument || parentIframes[i].contentWindow.document;
          if( iframeDoc === self.document ) {
              iframe = parentIframes[i];
              break;
          }
      }

      if( iframe ) {
          setTransparentBackground(iframeDoc);
          iframe.allowTransparency = 'true';
          self = parent;
          parent = self.parent;
      } else {
          break;
      }
  }
}, false);