有两个地方不懂，求解释

<form id="Input">

        <table align="center" width="500px" height="50%" border="1">

        <tr>

            <td align="center" width="100px">学号：</td>

            <td align="center" width="300px">

                <input type="text" id="userid" name="user" onblur="Validate()">

                <div id="usermsg"></div>

            </td>

        </tr>

        <tr>

            <td align="center" width="100px">姓名：</td>

            <td align="center" width="300px">

                <input type="text" name="name">

            </td>

        </tr>

        <tr>

            <td align="center" width="45%">性别：</td>

            <td align="center" width="300px">

            <input type="radio" name="sex" value="男">男

            <input type="radio" name="sex" value="女">女

            </td>

        </tr>   

        <tr>

            <td align="center" width="30%">年龄：</td>

            <td align="center" width="300px">

                <input type="text" name="age">

            </td>

        </tr>

        <tr>

            <td align="center" width="100px">地址：</td>

            <td align="center" width="300px">

                <input type="text" name="addr">

            </td>

        </tr>

        </table>

1. <div id=usermsg></div>
   这句是什么意思，有什么作用

2. <td align="center" width="100px">姓名:</td>

   <td align="center" width="45%">性别:</td>

   <td align="center" width="30%">年龄:</td>

   这几个设置不同宽度有什么意义，尤其是45%和30%运行出来宽度怎么还是一样的，求大神解释