Bad value “” for attribute “rowspan” on element “td”: The empty string is not a valid non-negative integer.

The rowspan attribute on a <td> must hold a whole number from 0 to 65534, so leaving it empty is an error.

A bare rowspan with no value counts as empty and gets the same message. A value made of spaces, or a negative number like -1, gets a different error that starts with "Expected a digit".

Empty rowspan attributes often come from table plugins and page builders that write both span attributes on every cell and leave them blank when no cells were merged. Check the same cells for colspan="", which the validator reports in its own message ending in "not a valid positive integer".

Browsers read an empty rowspan as 1, the same as no attribute, so deleting it leaves the table unchanged. Don't reach for rowspan="0" as a replacement: it is valid, unlike colspan="0", but it stretches the cell down through every remaining row of its <thead>, <tbody> or <tfoot>. Write the attribute only on cells that cover two or more rows.

Invalid example

<table>
  <tr>
    <th>Day</th>
    <th>Time</th>
    <th>Session</th>
  </tr>
  <tr>
    <td rowspan="2">Monday</td>
    <td rowspan="">9:00</td>
    <td rowspan="">Welcome</td>
  </tr>
  <tr>
    <td>11:00</td>
    <td>Workshop</td>
  </tr>
</table>

Valid example

<table>
  <tr>
    <th>Day</th>
    <th>Time</th>
    <th>Session</th>
  </tr>
  <tr>
    <td rowspan="2">Monday</td>
    <td>9:00</td>
    <td>Welcome</td>
  </tr>
  <tr>
    <td>11:00</td>
    <td>Workshop</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The table data cell element

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WHATWG Spec

HTML Standard: Tabular data

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