Q:

th colspan fails w3c validations

what is wrong with the following?
 <table>
 <tr> <th> Blah </th> <th colspan="2"> Something </th> </tr>
 <tr> <td> .. </td> <td colspan="2"> ... </td>  </tr>
 </table>

It says Table column 3established by element th has no cells beginning in it.

A:

As the error message says, there is no cell that begins in the third column. It thus violates the HTML table model as defined in HTML5, rather technically in 4.9.12 Processing model. Basically, the point is that you cannot create a column that consists only of slots created by cells starting in earlier columns and extending to other columns with colspan.
When using earlier versions of HTML, the error won’t be caught, since for them, validation is DTD-driven, and a DTD (document type definition, a formalized set of syntax rules written in SGML or XML) can only describe relatively simple syntax rules.
The most common cause of this problem seems to be a misguided idea of just making cells wider by using colspan. Instead, use the width attribute or, usually more preferably, the width property in CSS.

A:

I've validated your HTML code with two doctypes: 

HTML5;
XHTML 1.0 Strict.

And your document was successfully checked as valid. 
You should try to revalidate it.

