table-headers-attr-valid

Table cell "headers" attribute must reference valid header cells

Checks that each id in a <td>/<th> headers attribute resolves to a cell (<td> or <th>) within the same table (not missing, not a non-cell element, not itself).

Applies to. Applies to <td>/<th> elements that carry a non-empty headers attribute, within a <table> whose semantic role is still table/grid/treegrid -- an explicit role of anything else (role="presentation"/"none", but also role="heading" or any other real role) replaces the native table semantics, leaving no table for headers to describe. Matches ACT a25f45's applicability.

Expectation. Every id token in the headers attribute resolves to an element that: (a) exists, (b) is a cell (<td> or <th>) of the same <table> as the referencing cell, and (c) is not the cell itself. A <td> serving as a header via role="columnheader"/"rowheader" is a valid target, same as a plain <th> -- ACT a25f45 does not require the native tag.

Examples

<table>
  <tr><th id="h1">Name</th></tr>
  <tr><td headers="h1">Apple</td></tr>
</table>

<table>
  <tr><th id="h2">Name</th></tr>
  <tr><td headers="missing">Banana</td></tr>
</table>

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