See W3C documentation for the XHTML table model.
Horizontal spanning. The attribute specifies the number of columns that a table cell will span (The table cell will be tagged
with a <td> or <th> element.)
| Value |
Meaning |
| Counting number |
How many column widths this cell will span, e.g., “3”
|
| Default value: 1 (Spans 1 column)
|