[Dilettante_Pro]

Работает

<table>
   <tr>
      <td>11111</td>
      <td class="ms-vb-lastCell ms-cellstyle ms-vb2 ms-vb-lastCell">70</td>
   </tr>
   <tr>
      <td>22222</td>
      <td class="ms-vb-lastCell ms-cellstyle ms-vb2 ms-vb-lastCell">20</td>
   </tr>
   <tr>
      <td>33333</td>
      <td class="ms-vb-lastCell ms-cellstyle ms-vb2 ms-vb-lastCell">10</td>
   </tr>   
</table>
<script>
let td = document.getElementsByClassName('ms-vb-lastCell');
let sum = 0;


for(let i = 0; i < td.length; i++){
	sum += +td[i].innerHTML;
};
alert(sum);
</script>