ie9以下浏览器不兼容placeholder的解决方案

html结构如下：

<form action="post" class="xz-form" id="xz-form">
    <div id="validate"  class="xz-validate-tip"></div>

    <div class="xz-userName">
        <input class="xz-inputName"  id="userName" type="text" required="required" placeholder="用户名："/>
        <span class="hide xz-placeholder">用户名：</span>
        <img class="xz-info-wrong" src="images/wrong.png" alt="错误标志"/>
    </div>

    <div class="xz-password">
        <input class="xz-inputPwd" id="password" type="password" required="required" placeholder="密码："/>
        <span class="hide xz-placeholder">密码：</span>
        <img class="xz-info-wrong" src="images/wrong.png" alt="错误标志"/>
    </div>

    <div class="xz-verification">
        <input class="xz-inputVerification" id="verification" required="required" placeholder="验证码："/>
        <span class="hide xz-placeholder">验证码：</span>
        <img class="img-verification" src="images/getimg.jpg" alt="验证码图片"/>
        <img class="xz-info-wrong" src="images/wrong.png" style="right: 170px" alt="错误标志"/>

        <div class="clear"></div>
    </div>
  
    <div class="xz-btn-login">
        <a id="xz-btn" href="javascript:void(0)"  class="disabled">登录</a>
    </div>
</form>

js代码如下：

//检测是否是Ie浏览器
function isIE() { //ie?
    var browser=navigator.appName

    var b_version=navigator.appVersion

    var version=b_version.split(";");

    var trim_Version=version[1].replace(/[ ]/g,"");

    if(browser=="Microsoft Internet Explorer" && trim_Version=="MSIE7.0")
    {
        return true;
    }else if(browser=="Microsoft Internet Explorer" && trim_Version=="MSIE8.0"){
        return true;
    }else if(browser=="Microsoft Internet Explorer" && trim_Version=="MSIE9.0"){
        return true;
    } else {
        return false;
    }
}

//解决ie不支持placeholder
$('#xz-form span.xz-placeholder').each(function () {
    if(isIE()) {
        var $this = $(this);
        if(!$this.prev().val() || $this.prev().val() == "" ) {
            $this.show();
        }
    }
})

$('#xz-form span.xz-placeholder').bind("click",function(){
    if(isIE()){
        $(this).hide();
        $(this).prev().focus();
    }
})

$('#xz-form input').bind({
    focus:focus,
    blur:blur
})

function focus() {
    if(isIE()) {
        var $this = $(this);
        $this.next().hide();
    }
}

function blur() {
    if(isIE()){
        var $this = $(this);
        if($this.val() == ""){
            $this.next().show();
        }
    }
}