Information can be presented online in a number of visual ways. Tables offer a way of displaying data in a format that allows users to clarify relationships visually.
Their biggest advantage is in allowing users to cross reference two pieces of information to perceive a third piece of information, while ignoring everything else in the table.
That advantage is lost to users of screen readers, as they process content according to marked up instructions, not visually. Without specific markup, screen readers will read and voice all the table content left to right, top to bottom, thereby defeating the purpose of using a table at all.
HTML can be used to provide instructions for screen readers to follow, such as how to voice information presented in a table, and in what order information should be read. This way, markup helps preserve meaning and provides context for people using screen readers to access tabular data.
As an example, online bus schedules published as data tables can be marked up appropriately so that a person who uses a screen reader can match their bus route with time and place.
Most web professionals are aware that tables should be used only for the presentation of tabular data. They will be equally aware that tables have been used for many years now — and continue to be used — purely to assist in managing and controlling elements in a webpage layout.
The reality, as the WCAG 1.0 Note on Tables acknowledges, is that we need to know how to make all tables accessible, whether used for tabular data presentation or to manage page layout.