See W3C documentation for the XHTML table model.
Specifies the vertical position of data within a table cell, for example, flush to the top of the cell
|All cells in the same row as a cell whose valign attribute has this value should have their textual data positioned so that the first text line occurs on a baseline common to all cells in the row. This constraint does not apply to subsequent text lines in these cells.
|Cell data are flush with the bottom of the cell.
|Cell data are centered vertically within the cell.
|Cell data are flush with the top of the cell.
|Restriction: This attribute may be specified if the element is used.