Bad value “” for attribute “colspan” on element “th”: The empty string is not a valid positive integer.

A colspan on a <th> has to say how many columns the header covers, as a whole number from 1 to 1000, so colspan="" is invalid.

Writing colspan with no value at all gives the same message. If the value holds only spaces, as in colspan=" ", the validator reports it differently, with a message that begins "Expected a digit".

Header cells often end up like this when a table plugin or theme writes the span attributes on every <th> and fills them from a setting that is blank for most columns. Check the same headers for rowspan="". The validator flags it separately, as "not a valid non-negative integer", because rowspan also accepts 0.

An empty colspan changes nothing on screen: browsers read it as 1, just as they do when the attribute is missing. Remove it from every header that sits over a single column, and keep a number only where a header groups several columns, such as colspan="2" above two related columns.

Invalid example

<table>
  <tr>
    <th colspan="">Course</th>
    <th colspan="2">Schedule</th>
  </tr>
  <tr>
    <td>Biology</td>
    <td>Monday</td>
    <td>Wednesday</td>
  </tr>
</table>

Valid example

<table>
  <tr>
    <th>Course</th>
    <th colspan="2">Schedule</th>
  </tr>
  <tr>
    <td>Biology</td>
    <td>Monday</td>
    <td>Wednesday</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The table header element

→

WHATWG Spec

HTML Standard: Tabular data

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