See W3C documentation for the XHTML table model.
Defines which rules to draw between the rows and columns of an XHTML table
If the @rules attribute has not been used, the presence or absence of rules will be based on the @border attribute. If
<th border="0">
or there is no @border attribute, then no rules are drawn. Otherwise, the value of the @rules attribute is assumed to be “all”. (The related table attribute @frame only concerns rules around the outside of the table.)
| Value | Meaning |
|---|---|
| all | Rules on all rows and columns |
| cols | Rules between columns |
| groups | Rules between groups |
| none | No rules in table |
| rows | Rules between rows |
| Restriction: This attribute may be specified if the element is used. | |