2.2.57 [CSS-Level2-2009] Section 17.5.1, Table layers and transparency
The specification states:
A "missing cell" is a cell in the row/column grid that is not occupied by an element or pseudo-element. Missing cells are rendered as if an anonymous table-cell box occupied their position in the grid.
Quirks Mode and IE7 Mode (All Versions)
Missing cells that have content are treated as anonymous table-rows, not cells.