HTMLTableCellElement: свойство rowSpan

<table>
  <thead>
    <tr>
      <th>Col 1</th>
      <th>Col 2</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>1</td>
      <td>1</td>
    </tr>
    <tr>
      <td rowspan="2">2</td>
      <td>2</td>
    </tr>
    <tr>
      <td>3</td>
      <td>3</td>
    </tr>
    <tr>
      <td>4</td>
      <td>4</td>
    </tr>
  </tbody>
</table>
<button id="increase">Increase rowspan</button>
<button id="decrease">Decrease rowspan</button>
<div>The second cell of the first column spans <output>2</output> row(s).</div>

// Obtain relevant interface elements
const row = document.querySelectorAll("tbody tr")[1];
const cell = row.cells[0];
const output = document.querySelectorAll("output")[0];

const increaseButton = document.getElementById("increase");
const decreaseButton = document.getElementById("decrease");

increaseButton.addEventListener("click", () => {
  cell.rowSpan = cell.rowSpan + 1;

  // Update the display
  output.textContent = cell.rowSpan;
});

decreaseButton.addEventListener("click", () => {
  cell.rowSpan = cell.rowSpan - 1;

  // Update the display
  output.textContent = `${cell.rowSpan == 0 ? "all remaining" : cell.rowSpan}`;
});