See documentation for the HTML 4.0 (XHTML) table model.
Horizontal offset from the alignment character specified by the char attribute; the percent of the current column width to the left of the (left edge of the) alignment character.
Alignment on a particular character requires that three attributes be set in the document:
|Number||Where to set the alignment. 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.|