ASP.NET 验证控件做注册验证特效

<script type="text/javascript">
function myKeyUp() {
var user = document.getElementById("user");
if (document.getElementById("txtUser").value != "") {
user.style.display = "none"; //隐藏
}
else {
user.style.display = ""; //显示
}
}
</script>
<table id="mytable" cellspacing="0" style="width:100%">
<tr>
<td class="alt" style="width:75px">
账号
</td>
<td class="alt" style="width:175px">
<asp:TextBox ID="txtUser" runat="server"></asp:TextBox>
</td>
<td class="alt" style="width:305px">
<div id="user">
以字母开头,长度在6-18之间,只能包含字符、数字和下划线
</div>
<div>
<asp:RequiredFieldValidator ID="rfvUser" runat="server" ControlToValidate="txtUser"
ErrorMessage="必填" Display="Dynamic" ForeColor="Red"></asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="revUser" runat="server" Display="Dynamic"
ControlToValidate="txtUser" ErrorMessage="请检查格式是否正确" ValidationExpression="^[a-zA-Z]\w{5,17}$"
ForeColor="Red"></asp:RegularExpressionValidator>
</div>
</td>
</tr>
</table>

protected void Page_Load(object sender, EventArgs e)
{
txtUser.Attributes.Add("onchange", "myKeyUp();");
}