YUI Howto: Work with iFrames.

yui-iframe.js

YUI().use("node", function (Y) {
  'use strict';

  var iframeId = "#masterDataFrame";
  var frame = Y.one(iframeId);
  var win = Y.Node.getDOMNode(frame.get('contentWindow'));
  var doc = win.document;

  YUI({ win: win, doc: doc }).use('node', function (innerY) {
    // Do it...  
  });
});