The “columnheader” role is unnecessary for element “th”.

A <th> at the top of a column is already exposed as a column header, so role="columnheader" on it tells screen readers nothing new.

Browsers work out the ARIA role of a <th> from its place in the table. A header cell in the <thead>, or in a row of headers above the data, gets the role columnheader. A header cell at the start of a data row gets rowheader. When the layout leaves room for doubt, scope="col" or scope="row" settles it. Writing the role by hand adds nothing and makes the markup harder to read.

The validator reports this warning for role="columnheader" on any <th>, wherever it sits. On a header that starts a row, the role is also wrong, because it overrides rowheader and tells screen readers the cell heads a column.

In most tables you'll see an error next to this warning. When the <table> has no role attribute, or has role="table", role="grid" or role="treegrid", its <th> cells can't have a role attribute at all. Removing role="columnheader" fixes both messages.

Table plugins and markup copied from online word processors often add a role to every part of a table. The columnheader role exists for tables built from generic elements such as <div>, where nothing else marks a cell as a header. A native <th> doesn't need it, so delete the attribute and keep scope if you want to state the direction. If the same table has roles on its rows or other cells, remove those as well.

Invalid example

<table>
  <thead>
    <tr>
      <th scope="col" role="columnheader">Day</th>
      <th scope="col" role="columnheader">Opening hours</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Monday</td>
      <td>9:00 to 17:00</td>
    </tr>
    <tr>
      <td>Saturday</td>
      <td>10:00 to 14:00</td>
    </tr>
  </tbody>
</table>

Valid example

<table>
  <thead>
    <tr>
      <th scope="col">Day</th>
      <th scope="col">Opening hours</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Monday</td>
      <td>9:00 to 17:00</td>
    </tr>
    <tr>
      <td>Saturday</td>
      <td>10:00 to 14:00</td>
    </tr>
  </tbody>
</table>

Learn more:

MDN Reference

ARIA: columnheader role - MDN Web Docs

→

MDN Reference

The Table Header element - MDN Web Docs

→

W3C Reference

ARIA in HTML

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