for reference:
http://www.w3.org/TR/html401/struct/tables.html#h-11.2.6#
#rowspan = number [CN]
#This attribute specifies the number of rows spanned by the current cell. The
#default value of this attribute is one ("1"). The value zero ("0") means
#that the cell spans all rows from the current row to the last row of the
#table section (THEAD, TBODY, or TFOOT) in which the cell is defined.
#
#colspan = number [CN]
#This attribute specifies the number of columns spanned by the current cell.
#The default value of this attribute is one ("1"). The value zero ("0") means
#that the cell spans all columns from the current column to the last column
#of the column group (COLGROUP) in which the cell is defined.
#
this does not explicitely cover the case where no colgroup is defined, but it
seems safe to assume, given the behaviour for rowspan, that the cell should
span all the table section then.
