Mono Tips

I run an asp.net project on Ubuntu using Mono. I get below error on one page: 

I started to examine related part and I noticed the problem. Here is a sample:

<table id="tbltest" runat="server">
    <colgroup>
        <col style="width:50px;" />
        <col style="width:100px;" />
    </colgroup>
    <tbody>
        <tr>
            <td>cell1</td>
            <td>cell2</td>
        </tr>
        <tr>
            <td>cell3</td>
            <td>cell4</td>
        </tr>
        <tr>
        </tr>
    </tbody>
</table>

It fixed when I removed runat="server" from table. Another solution is removing colgroup parts.
Even I found a solution, I didn't understand why this is firing error.