If you want interior rules between your rows and columns for an XHTML table and you want this information expressed in the XML encoding , best practice is to set your desired rules explicitly using the @rules attribute. The nominal default for rules is “none”, but the setting for @rules can be changed by the values of other table attributes.

If the @rules attribute has NOT been set explicitly, the presence or absence of interior rules should be determined by other table attributes (as this is a display engine issue, your mileage may vary):

• If there is no @border attribute or @border has been set to zero (border="0"), @frame will be set to “void” and the @rules default (which you can override) will be set to “none”.
• if @border has any value other than zero, the default @frame will be set to “border” and the @rules default (which you can override) will be set to “all”.

The XHTML constraints on rules and frames have not changed much since HTML 4.1 in 1998, but browser and display behavior has changed over time. As of 2021, HTML5 recommendations deprecate @rules and instead suggest using CSS attributes to create the rules and frame for tables.