See documentation for the HTML 4.0 (XHTML) table model or OASIS (CALS) Exchange table model (XML version), dependent upon element involved.
Specifies the vertical position of data within a cell
|baseline||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.|
|bottom||Cell data is flush with the bottom of the cell.|
|middle||Cell data is centered vertically within the cell.|
|top||Cell data is flush with the top of the cell.|
|Restriction: This attribute may be specified if the element is used.|