2.1.84 [CSS-Level2-2009] Section 17.1, Introduction to tables
The specification states:
Authors may specify the visual formatting of a table as a rectangular grid of cells. Rows and columns of cells may be organized into row groups and column groups. Rows, columns, row groups, column groups, and cells may have borders drawn around them (there are two border models in CSS 2.1). Authors may align data vertically or horizontally within a cell and align data in all cells of a row or column.
Quirks Mode and IE7 Mode (All Versions)
The border property is not supported for the following elements: