TextBox控件里提示字符地隐藏显示

<script language="javascript">
   function checkLogin(n) {
   var userName = document.getElementById("userName")
   if (n == 0 && userName.value == "请在此输入"){userName.value="";userName.style.cssText = "color:#000"}
   if (n == 1 && userName.value == ""){userName.value = "请在此输入";userName.style.cssText = "color:#aaa"}
  
}
<form id="Form1" method="post" runat="server">
<asp:TextBox id="userName" runat="server"  ForeColor=Gainsboro    text= "请在此输入"></asp:TextBox>
</form>
-------------------隐藏文件地代码
if (!IsPostBack)
         {
            userName.Attributes.Add("onclick","this.focus();checkLogin(0)");
            userName.Attributes.Add("onblur","checkLogin(1)");           
         }