Markup shall be used to associate data cells and header cells for data tables that have two or more logical levels of row or column headers

Why this is important: Complex data tables are those in which there is more than one level of header for any column or row. All of the simple table information also applies to these. For text readers to work properly on multiple levels of headings, id= attributes must be added to the <th> elements, and matching headers= attributes to each data cell. Additional markup such as the <thead> element can be used to further clarify the table structure.