Element.ariaColCount

<table id="semantic-table" role="table" aria-label="Semantic Elements" aria-describedby="semantic_elements_table_desc" aria-rowcount="100" aria-colcount="2">
  <caption id="semantic_elements_table_desc">Semantic Elements to use instead of ARIA's roles</caption>
  <thead role="rowgroup">
    <tr role="row">
      <th role="columnheader" aria-sort="none" aria-rowindex="1">ARIA Role</th>
      <th role="columnheader" aria-sort="none" aria-rowindex="1">Semantic Element</th>
    </tr>
  </thead>
  <tbody role="rowgroup">
    <tr role="row">
      <td role="cell" aria-rowindex="11">header</td>
      <td role="cell" aria-rowindex="11">h1</td>
    </tr>
    <tr role="row">
      <td role="cell" aria-rowindex="16">header</td>
      <td role="cell" aria-rowindex="16">h6</td>
    </tr>
    <tr role="row">
      <td role="cell" aria-rowindex="18">rowgroup</td>
      <td role="cell" aria-rowindex="18">thead</td>
    </tr>
    <tr role="row">
      <td role="cell" aria-rowindex="24">term</td>
      <td role="cell" aria-rowindex="24">dt</td>
    </tr>
  </tbody>
</table>

let el = document.getElementById('semantic-table');
console.log(el.ariaColCount); // 2
el.ariaColCount = "3"
console.log(el.ariaColCount); // 3