If you use more than one method to add the same attribute to a table, you might end up with duplicate attribute assignments, which are not allowed for W3C validation. For example, suppose you specify access method scope for all tables (see §35.1.3.2 Applying the scope method to all tables ):
[Tables] AccessMethod=Scope
Then if you happen to include a CellScope PI marker in some table (see §35.2.4 Using PI markers for table-cell attributes0, the scope attribute assignment appears twice in the output for that table cell.