Have a look at this code, in responsive mode (taken from w3schools, example on rowspan):

 <tr> <th>Month</th> <th>Savings</th> <th>Savings for holiday!</th> </tr> <tr> <td>January</td> <td>$100</td> <td rowspan="2">$50</td> </tr> <tr> <td>February</td> <td>$80</td> </tr>

What do you think about it? Should the third th be shown also in the second row? Maybe yes, but I'm not so sure.