需求，会员注册，勾选统一注册协议复选框，按钮变 亮可用，
不勾选时 变灰不可用,以下是解决方案及原理：
ie7 及以下版本 --> alert(typeof($("#agree").attr("checked")));返回的是boolean类型
ie8 --> alert(typeof($("#agree").attr("checked"))); 如果没有勾选,提示undefined
勾选之后提示 string
ie9 或者以上版本 --> alert(typeof($("#agree").attr("checked"))); 均返回string类型
alert($("#agree").attr("checked"));勾选时返回 checked
agree 复选框ID
toubiao_submit 按钮ID
function agree_change(){
//IE8 或以上版本浏览器
if(typeof($("#agree").attr("checked"))=="undefined" || typeof($("#agree").attr("checked"))=="string"){
if(typeof($("#agree").attr("checked"))=="undefined")
{
$("#agree").attr("checked","checked");
$("#toubiao_submit").attr("disabled",false);
$("#toubiao_submit").removeClass("toubiao_btn_disable");
$("#toubiao_submit").addClass("toubiao_btn_able");
}else{
if($("#agree").attr("checked")=="checked"){
$("#agree").removeAttr("checked");
$("#toubiao_submit").attr("disabled",true);
$("#toubiao_submit").removeClass("toubiao_btn_able");
$("#toubiao_submit").addClass("toubiao_btn_disable");
}else{
$("#agree").attr("checked","checked");
$("#toubiao_submit").attr("disabled",false);
$("#toubiao_submit").removeClass("toubiao_btn_disable");
$("#toubiao_submit").addClass("toubiao_btn_able");
}
}
return;
}
//IE7 或以下版本浏览器
if(typeof($("#agree").attr("checked"))=="boolean"){
if($("#agree").attr("checked"))
{
$("#toubiao_submit").attr("disabled",false);
$("#toubiao_submit").removeClass("toubiao_btn_disable");
$("#toubiao_submit").addClass("toubiao_btn_able");
}else{
$("#toubiao_submit").attr("disabled",true);
$("#toubiao_submit").removeClass("toubiao_btn_able");
$("#toubiao_submit").addClass("toubiao_btn_disable");
}
return;
}
}