HTMLTableCellElement: colSpan property

<table>
  <thead>
    <tr>
      <th>Col 1</th>
      <th>Col 2</th>
      <th>Col 3</th>
      <th>Col 4</th>
      <th>Col 5</th>
      <th>Col 6</th>
      <th>Col 7</th>
      <th>Col 8</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td colspan="2">1</td>
      <td>2</td>
      <td>3</td>
      <td>4</td>
      <td>5</td>
      <td>6</td>
      <td>7</td>
      <td>8</td>
    </tr>
  </tbody>
</table>
<button id="increase">Increase colspan</button>
<button id="decrease">Decrease colspan</button>
<div>The first cell spans <output>2</output> column(s).</div>

table {
  border-collapse: collapse;
}

th,
td,
table {
  border: 1px solid black;
}

button {
  margin: 1em 1em 1em 0;
}

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

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

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

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

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

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