It is similar to your table
<table border=1 width=50%>
<tr>
<td rowspan="2">x</td>
<td colspan="4">y</td>
</tr>
<tr>
<td bgcolor=#FFFF00 >I</td>
<td>II</td>
<td bgcolor=#FFFF00>III</td>
<td>IV</td>
</tr>
<tr>
<td>empty</td>
<td bgcolor=#FFFF00>1</td>
<td>2</td>
<td bgcolor=#FFFF00>3</td>
<td>4</td>
</tr>