I found some "solution".
This code doesn't work:
<table>
<thead>
<tr>
<th colspan="3">Test</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
</tbody>
But this is ok:
<table>
<thead>
<tr>
<th colspan="2">Test</th>
<th></th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
</tbody>
I think, that the problem is, that the last TH can't have attribute colspan.