iframe嵌入网页随屏幕放大缩小

我有个网页test.html，然后在这里面嵌入了一个iframe，iframe资源是网页a.html；如何让test.html，放大缩小拉伸的时候，iframe里的网页也跟着test.html进行等比缩放

function SetCwinHeight(obj)
{
  var cwin=obj;
  if (document.getElementById)
  {
    if (cwin && !baiwindow.opera)
    {
      if (cwin.contentDocument && cwin.contentDocument.body.offsetHeight)
        cwin.height = cwin.contentDocument.body.offsetHeight; 
      else if(cwin.Document && cwin.Document.body.scrollHeight)
        cwin.height = cwin.Document.body.scrollHeight;
    }
  }
}
 
//引用时du
onload="javascript:SetCwinHeight(this)"