Headers attribute specified on a cell refers to cells in the same table element

Description

This rule checks that the headers attribute on a cell refer to other cells in the same table element.

Applicability

This rule applies to any headers attribute specified on a cell within a table element, where all of the following is true for the table element:

The table is visible; and
The table is included in the accessibility tree; and
The table has a semantic role of table, grid or treegrid.

Expectation 1

Each target’s attribute value is a set of space separated tokens. Each token is the value of the id attribute of an element, that is a cell of the same table.

Expectation 2

Each target’s attribute value is a set of space separated tokens, and none of these tokens is the id of the element on which the test target is specified.

Assumptions

This rule assumes that the headers attribute is only used to identify table headers. If other information is included in the headers attribute, the rule may fail on issues that are not accessibility concerns. For example, if headers is used to include information for scripts, this rule may not be accurate.
This rule assumes that the headers attribute is required to express the relationship between data and table header cells in the same table. If the browser computes an adequate fallback header for cells that have the headers attribute value that does not correspond to the id of any one cell in the same table, success Criterion 1.3.1 Info and Relationships may be satisfied even if this rule failed.
This rule assumes that the id values on the headers attribute are unique.

Accessibility Support

There are no accessibility support issues known.

Background

Bibliography

Understanding Success Criterion 1.3.1: Info and Relationships
H43: Using id and headers attributes to associate data cells with header cells in data tables
F90: Incorrectly associating table headers and content via the headers and id attributes
headers attribute referencing elements that are non-existent or not in the table are ignored when assigning header cells (step 3, first case, point 2).
headers attribute referencing to the cell itself are ignored when assigning header cells (step 3, first case, point 2).

Accessibility Requirements Mapping

1.3.1 Info and Relationships (Level A)
- Learn more about 1.3.1 Info and Relationships
- Required for conformance to WCAG 2.0 and later on level A and higher.
- Outcome mapping:
  - Any failed outcomes: success criterion is not satisfied
  - All passed outcomes: success criterion needs further testing
  - An inapplicable outcome: success criterion needs further testing
H43: Using id and headers attributes to associate data cells with header cells in data tables
- Learn more about technique H43
- Not required for conformance to any W3C accessibility recommendation.
- Outcome mapping:
  - Any failed outcomes: technique is not satisfied
  - All passed outcomes: technique needs further testing
  - An inapplicable outcome: technique needs further testing

Input Aspects

The following aspects are required in using this rule.

Test Cases

Passed

Passed Example 1

Open in a new tab

The headers attribute on the data cells refers to a th element within the same table.

<table>
	<thead>
		<tr>
			<th id="header1">Projects</th>
			<th id="header2">Objective</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td headers="header1">15%</td>
			<td headers="header2">10%</td>
		</tr>
	</tbody>
</table>

Passed Example 2

Open in a new tab

The headers attribute on the cell refers to a th element within the same table. Multiple headers are referenced for a cell with colspan of 2.

<table>
	<thead>
		<tr>
			<th id="header1">Projects</th>
			<th id="header2">Exams</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td colspan="2" headers="header1 header2">15%</td>
		</tr>
	</tbody>
</table>

Passed Example 3

Open in a new tab

The headers attribute on the data cells in the second row refers to a td element with a role of columnheader within the same table.

<table>
	<thead>
		<tr>
			<td role="columnheader" id="header1">Projects</td>
			<td role="columnheader" id="header2">Objective</td>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td headers="header1">15%</td>
			<td headers="header2">10%</td>
		</tr>
	</tbody>
</table>

Passed Example 4

Open in a new tab

This table has multiple elements with a role of columnheader. The headers attribute on the cells lists IDs of th elements within the same table.

<table>
	<tr>
		<th colspan="2" id="header1">Projects</th>
		<th colspan="2" id="header2">Exams</th>
	</tr>
	<tr>
		<th id="e1" headers="header1">1</th>
		<th id="e2" headers="header1">2</th>
		<th id="p1" headers="header2">1</th>
		<th id="p2" headers="header2">2</th>
	</tr>
	<tr>
		<td colspan="2" headers="header1 e1 e2">15%</td>
		<td headers="header2 p1">15%</td>
		<td headers="header2 p2">45%</td>
	</tr>
</table>

Passed Example 5

Open in a new tab

The headers attribute on the second data cell in each row refers to a th element with a role of rowheader within the same table.

<table>
	<tbody>
		<tr>
			<th role="rowheader" id="headerAge">Age</th>
			<td headers="headerAge">65</td>
		</tr>
		<tr>
			<th role="rowheader" id="headerObjective">Objective</th>
			<td headers="headerObjective">40%</td>
		</tr>
	</tbody>
</table>

Passed Example 6

Open in a new tab

The headers attribute on the last two th elements refers to another th element within the same table. Here the column header has a span of two columns.

<table>
	<tr>
		<th id="name" colspan="2">Name</th>
	</tr>
	<tr>
		<th headers="name">Firstname</th>
		<th headers="name">Lastname</th>
	</tr>
</table>

Passed Example 7

Open in a new tab

The headers attribute on the cells refers to th elements which are row scoped & within the same table.

<table>
	<tr>
		<th id="projects1" scope="row">Projects</th>
		<th id="progress1" scope="row">Progress</th>
	</tr>
	<tr>
		<td headers="projects1">My Project</td>
		<td headers="progress1">15%</td>
	</tr>
</table>

Passed Example 8

Open in a new tab

The headers attribute on the cell refers to th element which is not the same column as the cell.

<table>
	<tr>
		<td></td>
		<th id="projects2">Projects</th>
	</tr>
	<tr>
		<td headers="projects2">15%</td>
		<td></td>
	</tr>
</table>

Failed

Failed Example 1

Open in a new tab

The td elements have a headers attribute referring to an ID that does not exist within the same table. Here the referenced ID is incorrect.

<table>
	<tr>
		<th id="headerOfColumn1">Projects</th>
		<th id="headerOfColumn2">Objective</th>
	</tr>
	<tr>
		<td headers="headOfColumn1">15%</td>
		<td headers="headOfColumn2">10%</td>
	</tr>
</table>

Failed Example 2

Open in a new tab

The td elements have a headers attribute referring to an ID that exist in a separate table.

<table>
	<tr>
		<th id="headOfColumn1">Projects</th>
		<th id="headOfColumn2">Objective</th>
	</tr>
</table>

<table>
	<tr>
		<td headers="headOfColumn1">15%</td>
		<td headers="headOfColumn2">10%</td>
	</tr>
</table>

Failed Example 3

Open in a new tab

The td element has a headers attribute referring to its own ID.

<table>
	<tr>
		<th>Event Type</th>
	</tr>
	<tr>
		<td id="headerBday" headers="headerBday">
			Birthday
		</td>
	</tr>
</table>

Failed Example 4

Open in a new tab

The headers attribute on the data cells in the second row refers to an element inside the same table which does not have a role of rowheader or columnheader.

<table>
	<tr>
		<td>
			<span id="headerProject">Projects</span>
		</td>
		<td>
			<span id="headerObjective">Objective</span>
		</td>
	</tr>
	<tr>
		<td headers="headerProject">
			15%
		</td>
		<td headers="headerObjective">
			10%
		</td>
	</tr>
</table>

Inapplicable

Inapplicable Example 1

Open in a new tab

There is no headers attribute.

<table>
	<tr>
		<th scope="col">Projects</th>
		<th scope="col">Exams</th>
	</tr>
	<tr>
		<td>15%</td>
		<td>45%</td>
	</tr>
</table>

Inapplicable Example 2

Open in a new tab

The table has a role="presentation" and thus is not included in the accessibility tree.

<table role="presentation">
	<tr>
		<td id="header1">Project Status</td>
		<td id="header2">Objective</td>
	</tr>
	<tr>
		<td headers="header1">15%</td>
		<td headers="header2">10%</td>
	</tr>
</table>

Inapplicable Example 3

Open in a new tab

The table is not visible in page.

<html>
	<style>
		.notInPage {
			position: absolute;
			left: -9999px;
			top: -9999px;
		}
	</style>
	<table class="notInPage">
		<tr>
			<th id="header1">Project Status</th>
			<th id="header2">Objective</th>
		</tr>
		<tr>
			<td headers="header1">15%</td>
			<td headers="header2">10%</td>
		</tr>
	</table>
</html>

Inapplicable Example 4

Open in a new tab

The rule applies only to headers attribute within a table element.

<div role="table">
	<div role="row">
		<div role="columnheader" id="header1">Projects</div>
		<div role="columnheader" id="header2">Exams</div>
	</div>
	<div role="row">
		<div role="cell" headers="header2">15%</div>
		<div role="cell" headers="header1">15%</div>
	</div>
</div>

Inapplicable Example 5

Open in a new tab

The table is not included in the accessibility tree.

<table style="display:none;">
	<tr>
		<td id="header1">Project Status</td>
		<td id="header2">Objective</td>
	</tr>
	<tr>
		<td headers="header1">15%</td>
		<td headers="header2">10%</td>
	</tr>
</table>

Inapplicable Example 6

Open in a new tab

This table doesn’t have a role of table, grid or treegrid.

<table role="region">
	<td id="self" headers="self">World</td>
</table>

Glossary

Attribute value

The attribute value of a content attribute set on an HTML element is the value that the attribute gets after being parsed and computed according to specifications. It may differ from the value that is actually written in the HTML code due to trimming whitespace or non-digits characters, default values, or case-insensitivity.

Some notable case of attribute value, among others:

For enumerated attributes, the attribute value is either the state of the attribute, or the keyword that maps to it; even for the default states. Thus <input type="image" /> has an attribute value of either Image Button (the state) or image (the keyword mapping to it), both formulations having the same meaning; similarly, “an input element with a type attribute value of Text” can be either <input type="text" />, <input /> (missing value default), or <input type="invalid" /> (invalid value default).
For boolean attributes, the attribute value is true when the attribute is present and false otherwise. Thus <button disabled>, <button disabled="disabled"> and <button disabled=""> all have a disabled attribute value of true.
For attributes whose value is used in a case-insensitive context, the attribute value is the lowercase version of the value written in the HTML code.
For attributes that accept numbers, the attribute value is the result of parsing the value written in the HTML code according to the rules for parsing this kind of number.
For attributes that accept sets of tokens, whether space separated or comma separated, the attribute value is the set of tokens obtained after parsing the set and, depending on the case, converting its items to lowercase (if the set is used in a case-insensitive context).
For aria-* attributes, the attribute value is computed as indicated in the WAI-ARIA specification and the HTML Accessibility API Mappings.

This list is not exhaustive, and only serves as an illustration for some of the most common cases.

The attribute value of an IDL attribute is the value returned on getting it. Note that when an IDL attribute reflects a content attribute, they have the same attribute value.

Explicit Semantic Role

The explicit semantic role of an element is determined by its role attribute (if any).

The role attribute takes a list of tokens. The explicit semantic role is the first valid role in this list. The valid roles are all non-abstract roles from WAI-ARIA Specifications. If the element has no role attribute, or if it has one with no valid role, then this element has no explicit semantic role.

Other roles may be added as they become available. Not all roles will be supported in all assistive technologies. Testers are encouraged to adjust which roles are allowed according to the accessibility support base line. For the purposes of executing test cases in all rules, it should be assumed that all roles are supported by assistive technologies so that none of the roles fail due to lack of accessibility support.

Focusable

An element is focusable if one or both of the following are true:

the element is part of sequential focus navigation; or
the element has a tabindex value that is not null.

Exception: Elements that lose focus during a period of up to 1 second after gaining focus, without the user interacting with the page the element is on, are not considered focusable.

Notes:

The 1 second time span is an arbitrary limit which is not included in WCAG. Given that scripts can manage the focus state of elements, testing the focusability of an element consistently would be impractical without a time limit.
The tabindex value of an element is the value of the tabindex attribute parsed using the rules for parsing integers. For the tabindex value to be different from null, it needs to be parsed without errors.

Implicit Semantic Role

The implicit semantic role of an element is a pre-defined value given by the host language which depends on the element and its ancestors.

Implicit roles for HTML and SVG, are documented in the HTML accessibility API mappings (working draft) and the SVG accessibility API mappings (working draft).

Included in the accessibility tree

Elements included in the accessibility tree of platform specific accessibility APIs are exposed to assistive technologies. This allows users of assistive technology to access the elements in a way that meets the requirements of the individual user.

The general rules for when elements are included in the accessibility tree are defined in the core accessibility API mappings. For native markup languages, such as HTML and SVG, additional rules for when elements are included in the accessibility tree can be found in the HTML accessibility API mappings (working draft) and the SVG accessibility API mappings (working draft).

For more details, see examples of included in the accessibility tree.

Programmatically hidden elements are removed from the accessibility tree. However, some browsers will leave focusable elements with an aria-hidden attribute set to true in the accessibility tree. Because they are hidden, these elements are considered not included in the accessibility tree. This may cause confusion for users of assistive technologies because they may still be able to interact with these focusable elements using sequential keyboard navigation, even though the element should not be included in the accessibility tree.

Marked as decorative

An element is marked as decorative if one or more of the following conditions is true:

it has an explicit role of none or presentation; or
it is an img element with an alt attribute whose value is the empty string (alt=""), and with no explicit role.

Elements are marked as decorative as a way to convey the intention of the author that they are pure decoration. It is different from the element actually being pure decoration as authors may make mistakes. It is different from the element being effectively ignored by assistive technologies as rules such as presentational roles conflict resolution may overwrite this intention.

Elements can also be ignored by assistive technologies if they are programmatically hidden. This is different from marking the element as decorative and does not convey the same intention. Notably, being programmatically hidden may change as users interact with the page (showing and hiding elements) while being marked as decorative should stay the same through all states of the page.

Outcome

An outcome is a conclusion that comes from evaluating an ACT Rule on a test subject or one of its constituent test target. An outcome can be one of the three following types:

Inapplicable: No part of the test subject matches the applicability
Passed: A test target meets all expectations
Failed: A test target does not meet all expectations

Note: A rule has one passed or failed outcome for every test target. When there are no test targets the rule has one inapplicable outcome. This means that each test subject will have one or more outcomes.

Note: Implementations using the EARL10-Schema can express the outcome with the outcome property. In addition to passed, failed and inapplicable, EARL 1.0 also defined an incomplete outcome. While this cannot be the outcome of an ACT Rule when applied in its entirety, it often happens that rules are only partially evaluated. For example, when applicability was automated, but the expectations have to be evaluated manually. Such “interim” results can be expressed with the incomplete outcome.

Programmatically Hidden

An HTML element is programmatically hidden if either it has a computed CSS property visibility whose value is not visible; or at least one of the following is true for any of its inclusive ancestors in the flat tree:

has a computed CSS property display of none; or
has an aria-hidden attribute set to true

Note: Contrary to the other conditions, the visibility CSS property may be reverted by descendants.

Note: The HTML standard suggests setting the CSS display property to none for elements with the hidden attribute. While not required by HTML, all modern browsers follow this suggestion. Because of this the hidden attribute is not used in this definition. In browsers that use this suggestion, overriding the CSS display property can reveal elements with the hidden attribute.

Semantic Role

The semantic role of an element is determined by the first of these cases that applies:

Conflict If the element is marked as decorative, but the element is included in the accessibility tree; or would be included in the accessibility tree when it is not programmatically hidden, then its semantic role is its implicit role.
Explicit If the element has an explicit role, then its semantic role is its explicit role.
Implicit The semantic role of the element is its implicit role.

This definition can be used in expressions such as “semantic button” meaning any element with a semantic role of button.

Visible

Content perceivable through sight.

Content is considered visible if making it fully transparent would result in a difference in the pixels rendered for any part of the document that is currently within the viewport or can be brought into the viewport via scrolling.

Content is defined in WCAG.

For more details, see examples of visible.

WAI-ARIA specifications

The WAI ARIA Specifications group both the WAI ARIA W3C Recommendation and ARIA modules, namely:

Note: depending on the type of content being evaluated, part of the specifications might be irrelevant and should be ignored.

Rule Versions

Latest version, 31 August 2023 (compare)
- Update applicability to account for tables with a role attribute
Previous version, 23 June 2022

Implementations

This section is not part of the official rule. It is populated dynamically and not accounted for in the change history or the last modified date.

Implementation	Type	Consistency
Alfa (fully automated) 0.80.0	Automated tool	Consistent
Alfa (semi-automated) 0.80.0	Semi-automated tool	Consistent
Axe DevTools Pro 4.37.1	Semi-automated tool	Consistent
Axe-core 4.8.3	Automated tool	Consistent
Equal Access Accessibility Checker 3.1.42-rc.0	Automated tool	Consistent
QualWeb 3.0.0	Automated tool	Consistent
SortSite 6.45	Automated tool	Consistent
Total Validator 17.4.0	Linter	Consistent
Total Validator (+Browser) 17.4.0	Automated tool	Consistent