Determines data type of column (integer, floating point, or just strings). Sorting an already sorted column reverses the order.
inline via document popup
inline via column headers
Later might try to interpret dates. For now, LocalDirectory and Zip put elided sortable element at start of cell.
Not many nicely formed tables in the wild, but lotsa tabular data not in TABLE/TR/TD hierarchy, such as directory listings most web servers serve up. Extend to sort lines in section.