See documentation for the HTML 4.0 (XHTML) table model.
Width of space between cells in a table
| Value | Meaning |
|---|---|
| Number | Thickness, measured in pixels. Although the DTD can enforce only that this attribute contain any characters, validation beyond the DTD can constrain this value to a counting number (positive integer). |
| Restriction: This attribute may be specified if the element is used. | |