See W3C documentation for the 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:
|Where to set the alignment. 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.