为什么无法读取iframe远程页面的代码?

[laiyuehui]

<td><iframe src="http://www.sina.com.cn" name="mainframe" style="width:400; height:300; border-top-style:solid; border-top-color:red; border-left-style:solid; border-left-color:red; border-right-style:solid; border-right-color:blue; border-bottom-style:solid; border-bottom-color:green"></iframe>


window.mainframe.document.body.innerText;//此行访问源码出错.
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