See documentation for the HTML 4.0 (XHTML) table model.
Defines which rules to draw between the rows and columns of an XHTML table.
or there is no Border (Frame) Around a Table (XHTML table model) attribute, then no rules are drawn. Otherwise, the value of the Rules (XHTML table model) attribute is assumed to be “all”. (The related table attribute Frame (XHTML table model; OASIS table model) only concerns rules around the outside of the table.)