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WCAG: 1.3.1: All non-empty <td> elements in tables larger than 3 by 3 must have an associated table header
Data table markup can be tedious and confusing. Tables must be marked up done 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.
When tables are not marked up semantically and do not have the correct header structure, screen reader users cannot correctly perceive the relationships between the cells and their contents visually.