表单提交至iframe

.......
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
........
<form id="fabokeForm" method="GET" action="json.asp" accept-charset="GBK" target="hiddenIframe">
..... ......
</form>
<iframe  id="hiddenIframe" name="hiddenIframe" width=0 height=0 style="display:none" frameborder=0 src="about:blank"></iframe>

   var  form = document.getElementById("fabokeForm");

    /**/
    var iframe = document.getElementById("hiddenIframe");
    function iframeOnload(){
        var html = "",json = null;
        try{
            var el = (iframe.contentWindow || iframe.contentDocument.parentWindow).document.body;
            while(el &&  el.nodeType !== 3){
                el = el.firstChild;
            }
            html = el.nodeValue ;
            json = html ? eval("(" + html + ")") : false;
        }catch(e){}
        if(json){
            if(json.data && json.data.title){
                alert(json.data.title);
            }else if (json.error){
                alert(json.error || "上传失败。");
            }
        }
    }

    if(iframe.attachEvent){
        iframe.attachEvent("onload",iframeOnload);
    }else{
        iframe.onload = iframeOnload;
    }
    //处理页面编码问题
    if(navigator.userAgent.indexOf("MSIE") !== -1){
        form.onsubmit = function(e){
            document.charset='GBK';
        } 
        window.onbeforeunload = function(){
             document.charset='UTF-8';
        }
    }