The summary of a table is never displayed in visual browsers; it is exclusively designed for screen readers and speech browsers.

It is exactly what it sounds like: a summary, a longer description than the caption.

It is usually read immediately before the caption. Every table should have a summary. If you use tables for layout, you should give each of those tables an empty summary, to indicate that the table is used exclusively for visual layout and not for presenting tabular data. (This is a similar concept to providing an empty ALT attribute).

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