Table cell spans past the end of its row group established by a “thead” element; clipped to the end of the row group.

A cell inside a <thead> has a rowspan value larger than the number of rows left in that <thead>.

The <thead>, <tbody> and <tfoot> elements each form a row group, and a cell's rowspan can't reach from one row group into the next. When a header cell asks for more rows than its <thead> holds, the browser stops the cell at the last header row, as if the extra part of the rowspan weren't there.

This usually comes from a two-level header. The first row has a cell such as "Name" with rowspan="2" next to a grouped heading with colspan="2", and the row of sub-headings that belongs under the group ends up after </thead>, as the first row of the <tbody>. The "Name" cell can't extend into the <tbody>, so that row ends up one cell short, and the validator usually adds a second message about it: "A table row was 2 columns wide, which is less than the column count established by the first row (3)."

Put every header row inside the <thead> so the span has rows to cover. If the header really has a single row, remove the rowspan.

Invalid example

<table>
  <thead>
    <tr>
      <th rowspan="2">Name</th>
      <th colspan="2">Score</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>Round 1</th>
      <th>Round 2</th>
    </tr>
    <tr>
      <td>Ana</td>
      <td>8</td>
      <td>9</td>
    </tr>
  </tbody>
</table>

Valid example

<table>
  <thead>
    <tr>
      <th rowspan="2">Name</th>
      <th colspan="2">Score</th>
    </tr>
    <tr>
      <th>Round 1</th>
      <th>Round 2</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Ana</td>
      <td>8</td>
      <td>9</td>
    </tr>
  </tbody>
</table>

Learn more:

MDN Reference

The Table Head element - MDN Web Docs

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MDN Reference

The Table Header element - MDN Web Docs

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WHATWG Spec

HTML Standard: Tabular data

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