See W3C documentation for the XHTML table model.
Specifies the amount of space, in pixels, between the border of a cell and the contents of a table cell
|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.|