2.2.17 [CSS-Level2-2009] Section 17.5, Visual layout of table contents


The specification states:

 Cells may span several rows or columns… If this position would cause a column-spanning cell to overlap a row-spanning cell from a prior row, CSS does not define the results: implementations may either overlap the cells (as is done in many HTML 
 implementations) or may shift the later cell to the right to avoid such overlap. 

All Document Modes (All Versions)

If a column-spanning cell overlaps with a row-spanning cell, the cells are overlapped.