See documentation for the HTML 4.0 (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 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.|