表格的问题。。
不知道为什么表格出现断层了。。。，这里把html代码跟css代码都贴出来。。麻烦大家帮忙看下
<table width="100%" height="100%" border="0" cellpadding="0"
cellspacing="0" id="tb1">
<tr>
<td class="topbg" COLSPAN="2">
<img src=../images/ico/add.png border=0
style='margin-top: 5px; margin-bottom: -3px'>
作业详细信息
</td>
</tr>
<tr>
<td height="0" valign="top" COLSPAN="2">
<table border="0" cellspacing="0" cellpadding="0" CLASS="table">
<tr>
<td align="left" style="padding-left: 10px;" COLSPAN="4"
class="table_th">
<B>作业信息 </B>
</td>
</tr>
<tr>
<td width="12%" class="td_1">
作业标题
</td>
<td width="38%" class="td_2">
标题1
</td>
</tr>
<tr>
<td width="12%" class="td_1">
具体要求
</td>
<td width="38%" colspan="3" class="td_3">
test123

</td>
</tr>
<tr>
<td class="td_1" width="12%">
开始时间
</td>
<td class="td_2" idth="38%">
2012

</td>

</tr>

<tr>
<td class="td_1">
截止时间
</td>
<td class="td_2">
2012
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td width="48%">
<table border="0" cellspacing="0" cellpadding="0" CLASS="table1"
id="tb2">
<tr>
<td align="left" style="padding-left: 10px;" COLSPAN="4"
class="table_th">
作业上传列表
<input type="button" class="button" runat="server" value="打开"
onclick="SetView();return false;" />
<br />
</td>
</tr>
<tr>


<td class="td_1">
文件名
</td>
<td class="td_1">
<span>操作</span>
</td>
</tr>
</table>
</td>
<td width="48%">
<table border="0" cellspacing="0" cellpadding="0" CLASS="table1"
id="tb2">
<tr>
<td align="left" style="padding-left: 10px;" COLSPAN="4"
class="table_th">
教师反馈上传列表
<input type="button" class="button" runat="server" value="打开"
onclick="SetView();return false;" />
<br />
</td>
</tr>
<tr>


<td class="td_1">
文件名
</td>
<td class="td_1">
<span>操作</span>