See W3C documentation for the XHTML table model.
Specifies the thickness, in pixels, of the border to draw around the table. A value of “0” indicates that the table has no border. Supplying the @border attribute without specifying a value indicates that the table has a border of the application’s default thickness.
|Number||Thickness, measured in pixels, of the border around the table. Although the Tag Set can enforce only that this attribute contain any text, numbers, or special 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.|