headers Headers (NISO JATS table model)
The @headers attribute is intended to make XHTML tables more accessible by making explicit the relationship between a table header cell and the cells(s) to which it applies. Screen readers and other Assistive Technologies use @headers to provide information about the table structure that would be obvious to most sighted people.
Table headers are not always the <th> (table head) elements at the top of the table; for example, the cells in the first row of the table may be “headers” that relate to the rest of the cells in the current row.
The @headers attribute provides information about the table structure by letting each cell name (point to the @id attributes of) the table header cells or table data cells that are acting as the cell’s header. More than one header cell can be named in a @headers attribute, to name a span or both row and column headers.
@headers contains a list of the <td> and <th> elements that provide header information for the current table cell. If any of those elements also has a @headers attribute, the elements to which those @headers attributes refer also provide header information for the current cell. In HTML5, the table cells that are referred to in a “headers” attribute are “directly targeted”, and the table cells that are referred to in the “headers” attributes of those elements are “indirectly targeted” by the current table cell.
- In both HTML 4.01 and HTML5, “headers” is defined as referring to table header elements (<th> elements) only.
- HTML5 states that “headers” should only target elements in the same table, and that a table cell should not target itself (directly or indirectly).
- In PDF, the “Headers” attribute should refer only to “TH” structure elements.
- PDF 2.0 adds additional requirements that row IDs should be listed before column IDs and that row and column IDs should be listed from most specific to most general.
- WCAG 2.1 Technique H43 recommends using @id and @headers attributes to associate data cells with header cells in data tables.
- ISO 14289-1:2014 (PDF/UA) states that tables should include headers (small ‘h’) and that the headers should tagged according to the table in the PDF spec that defines the optional “Headers” attribute.
- The PDF/UA standard says that “Scope” shall be used when the table’s structure is not determinable from “Headers” and “IDs”, which seems to be an implicit assumption that “Headers” should be used.
- The Matterhorn Protocol failure condition 15-003 is for a table without “Headers” that does not have “Scope” attributes on “TH” structure elements, which implies that you need either “Headers” or “Scope”within a table.