对文本框设值时回上下抖动

Question

只是稍微上下抖动一些，不是特别明显，但不好看。
把设值的语句去掉，就不会抖动了，不知是怎么回事？
<asp:TextBox ID="txtUserName" Width="100" Text="请输入" ForeColor="DimGray" onFocus="javascript:if(this.value=='请输入'){this.value='';}"  onBlur="javascript:if(this.value==''){this.value='请输入';}" runat="server"></asp:TextBox>

Answer 1

可能我给的代码太少。。
                                        <table cellspacing="0" cellpadding="0" width="100%" border="0" height="328">
                                            <tr>
                                                <td height="164" colspan="2" align="middle">
                                                    <table cellspacing="0" cellpadding="0" width="100%" border="0" height="143" class="login_txt">
                                                        <tr>
                                                            <td style="width: 13%; height: 38; text-align: left">
                                                                <span>用户名：</span>
                                                            </td>
                                                            <td style="height: 38px; text-align: left">
                                                                <asp:TextBox ID="txtUserName" Width="100" Text="请输入" ForeColor="DimGray" onfocus="if(this.value=='请输入'){this.value='';}"  onblur="if(this.value==''){this.value='请输入';}" runat="server"></asp:TextBox>
                                                                还没有注册？
                                                             
                                                                <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" 
                                                                    ControlToValidate="txtUserName" Display="Dynamic" ErrorMessage="用户名不能为空！"></asp:RequiredFieldValidator>
                                                            </td>
                                                        </tr>
                                                        <tr>
                                                            <td style="width: 13%; height: 38; text-align: left">
                                                                <span>口&nbsp;&nbsp;&nbsp;&nbsp;令：</span>
                                                            </td>
                                                            <td style="height: 38px; text-align: left">
                                                                <asp:TextBox ID="txtPassWord" Width="100" runat="server" TextMode="Password"></asp:TextBox>
                                                                <img src="images/luck.gif">
                                                                忘记密码？
                                                            </td>
                                                        </tr>
                                                        <tr>
                                                            <td style="width: 13%; height: 35; text-align: left">
                                                                <span>验证码：</span>
                                                            </td>
                                                            <td style="height: 38px; text-align: left">
                                                                <asp:TextBox ID="txtVerifyCode" Width="100" runat="server"></asp:TextBox>
                                                                <img src="images/luck.gif">
                                                                看不清楚？
                                                            </td>
                                                        </tr>
                                                        <tr>
                                                            <td height="35">
                                                            </td>
                                                            <td height="35">
                                                                <asp:Button ID="btnLogin" runat="server" Text="登录" OnClick="btnLogin_Click" />
                                                                <asp:Button ID="btnReWrite" runat="server" Text="重填" OnClientClick="javascript:ClearText()" />
                                                            </td>
                                                </td>
                                            </tr>
                                        </table>