index->
<div class="d1">
<iframe src="a.html" frameborder="0" name="one" id="iframeId"></iframe>
</div>

<div id="dd">
<h1>iframe里的元素！</h1>
</div> 

var d=window.frames["one"].window;
d.onload=function(){
console.log(d.document.getElementById("dd"));
}; 

var ifr = document.createElement('iframe');
ifr.src = 'a.html';
document.body.appendChild(ifr);
ifr.onload = function(){
var doc = ifr.contentDocument || ifr.contentWindow.document;
// 在这里操纵b.html
console.log(doc.getElementById("dd"));
};