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

Every ID in the headers attribute of a th has to be the id of another th in the same table, and the validator flags each one that isn't.

On a header cell, headers ties a sub-header to the header cells above it or beside it. When a "Sales" header spans the "Q1" and "Q2" columns, <th headers="sales">Q1</th> says that Q1 belongs to Sales. The value is a space-separated list of IDs, and the validator checks each one on its own, so a single cell can trigger this message more than once.

The referenced header is usually where things went wrong. The validator reports the ID when:

• the group header has no id, or a different one (IDs are case-sensitive, so Sales doesn't match sales)
• the id sits on a td styled to look like a heading, or on the tr, the thead or a span inside the th, instead of on the th itself
• the th with that id belongs to another table, like a nested table or a separate table used as a fixed header row

Screen readers rely on these references to tell users which headers a cell falls under. When an ID doesn't resolve to a th, the sub-header loses its link to the group. Put the id on the group header's th, changing a td into a th if needed. If that header no longer exists, remove its ID from headers.

The standard also forbids a header cell that targets itself, either directly or through a chain of other headers' headers attributes. The validator doesn't report that loop, so check for it yourself when header cells point at each other.

Invalid example

<table>
  <tr>
    <td id="sales" colspan="2">Sales</td>
  </tr>
  <tr>
    <th headers="sales">Q1</th>
    <th headers="sales">Q2</th>
  </tr>
  <tr>
    <td>1200</td>
    <td>1350</td>
  </tr>
</table>

Valid example

<table>
  <tr>
    <th id="sales" colspan="2">Sales</th>
  </tr>
  <tr>
    <th headers="sales">Q1</th>
    <th headers="sales">Q2</th>
  </tr>
  <tr>
    <td>1200</td>
    <td>1350</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The table header element

→

WHATWG Spec

HTML Standard: Tabular data

→

W3C Reference

W3C WAI: Tables with multi-level headers

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