How to hide the border for specified rows of a table?

Use the CSS property border on the <td>s following the <tr>s you do not want to have the border.

In my example I made a class noBorder that I gave to one <tr>. Then I use a simple selector tr.noBorder td to make the border go away for all the <td>s that are inside of <tr>s with the noBorder class by assigning border: 0.

Note that you do not need to provide the unit (i.e. px) if you set something to 0 as it does not matter anyway. Zero is just zero.

table, tr, td {
  border: 3px solid red;
}
tr.noBorder td {
  border: 0;
}

<table>
  <tr>
    <td>A1</td>
    <td>B1</td>
    <td>C1</td>
  </tr>
  <tr class="noBorder">
    <td>A2</td>
    <td>B2</td>
    <td>C2</td>
  </tr>
  <tr>
    <td>A3</td>
    <td>A3</td>
    <td>A3</td>
  </tr>
</table>

Here’s the output as an image: