Bad value X for attribute “colspan” on element “th”: Expected a digit but saw Y instead.

The colspan attribute of a <th> takes a positive whole number written with digits only, and the validator names the first character in the value that isn't a digit.

colspan sets how many columns a header cell spans. The HTML standard requires a valid non-negative integer greater than zero and no larger than 1000, such as 2 or 3. Units, percentages, decimals, signs, spaces, quotes and keywords like auto all make the value invalid. The character after "saw" is where the number stopped: p in 2px, % in 100%, . in 1.5, - in -1.

A value that contains markup, like 2><span class=, usually means a quote is missing. In <th colspan="2><span class="label">, nothing closes the quote after 2, so the parser reads on to the next " and puts everything in between into colspan. The <span> start tag is lost, and what's left of it becomes stray attributes on the <th>. That's why errors like "No space between attributes" and "Quote in attribute name" usually appear next to this one. When the number is missing too, the value starts right at the >. A CMS or rich text editor can store the broken attribute with the < escaped as <, so check the saved content as well as the template.

Browsers read the leading digits and ignore the rest, so 2px still spans two columns. A value that doesn't start with a digit, or is zero, falls back to 1. The header then covers one column and every cell after it in the row moves over, so screen readers can announce data cells under the wrong header.

Write the number on its own, without a unit, and close the attribute's quote before the >.

Invalid example

<table>
  <tr>
    <th colspan="2px">Name</th>
    <th colspan="2><span class="label">Contact</span></th>
  </tr>
  <tr>
    <td>Ada</td>
    <td>Lovelace</td>
    <td>ada@example.com</td>
    <td>555-0100</td>
  </tr>
</table>

Valid example

<table>
  <tr>
    <th colspan="2">Name</th>
    <th colspan="2"><span class="label">Contact</span></th>
  </tr>
  <tr>
    <td>Ada</td>
    <td>Lovelace</td>
    <td>ada@example.com</td>
    <td>555-0100</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The table header element

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WHATWG Spec

HTML Standard: Tabular data

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WHATWG Spec

HTML Standard: Common microsyntaxes

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