See W3C documentation for the XHTML table model.
Width of space between cells in a table
| Value | Meaning |
|---|---|
| Number | Thickness, measured in pixels. Although the tag set can enforce only that this attribute contain any characters, validation beyond the DTD (for example, schema validation or Schematron validation) can constrain this value to a counting number (positive integer). |
| Restriction: This attribute may be specified if the element is used. | |