The “headers” attribute on the element “td” refers to the ID X, but there is no “th” element with that ID in the same table.

A td element’s headers attribute must reference the ID of a th element within the same table.

The headers attribute is used to define explicit associations between data cells (td) and header cells (th) by referencing header cell IDs. For valid markup, every ID referenced in the headers attribute must exist as a th element’s id within the same table. This helps users—especially those using assistive technology—understand which header(s) apply to each data cell.

For example, if a td references headers="table_header_1", there must be a th id="table_header_1" present. If it’s missing or mistyped, you’ll get the validator error you described.

Correct pattern:

<table>
  <tr>
    <th id="table_header_1">Header 1</th>
    <th id="table_header_2">Header 2</th>
  </tr>
  <tr>
    <td headers="table_header_1">Row 2, Column 1</td>
    <td headers="table_header_2">Row 2, Column 2</td>
  </tr>
</table>

Incorrect pattern (missing th ID):

<table>
  <tr>
    <th>Header 1</th>
    <th id="table_header_2">Header 2</th>
  </tr>
  <tr>
    <td headers="table_header_1">Row 2, Column 1</td>
    <td headers="table_header_2">Row 2, Column 2</td>
  </tr>
</table>

To resolve the error, ensure every value in a td‘s headers attribute matches the id of a th element in the same table.