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WCAG: 1.3.1: All cells in a <table> element that use the headers attribute must only refer to other cells of that same <table>
Data table markup can be tedious and confusing. Markup tables semantically and with the correct header structure. Screen readers have features to ease table navigation, but tables must be marked up accurately for these features to work correctly.
Screen readers have a specific way of announcing tables. When tables are not properly marked up, this creates the opportunity for confusing or inaccurate screen reader output.
Sighted users can usually tell at a glance what the table's headers are and what their relationship to the data is. For non-sighted users this must be done in the markup.
When a data table is designed with accessibility in mind, the user enters into table navigation mode, which allows the user to navigate from cell to cell within the table while hearing the screen reader announce the corresponding table headers for the data cells. Hearing the table headers is especially helpful when navigating through large data tables, or when cells contain similar-sounding data that could be easily confused.
Table navigation mode is not useful, though, if the table lacks accessibility features.