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WCAG 2 Test Rules

For developers of evaluation tools and test methodologies

About WCAG 2 Test Rules

Proposed id attribute value is unique

Description

This rule checks that all id attribute values on a single page are unique.

Applicability

This rule applies to any id attribute whose value is not an empty string (""), specified on a HTML or SVG element.

Note: Elements that are neither included in the accessibility tree nor visible are still considered for this rule.

Expectation

The value of the attribute is unique across all other id attributes specified on HTML or SVG elements that exist within the same document tree or shadow tree as the element on which the applicable id attribute is specified.

Assumptions

There are currently no assumptions.

Accessibility Support

There are no major accessibility support issues known for this rule.

Background

Bibliography

Accessibility Requirements Mapping

Input Aspects

The following aspects are required in using this rule.

Test Cases

Passed

Passed Example 1

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There is only one id attribute within the document context.

<div id="my-div">This is my first element</div>

Passed Example 2

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All id attributes are unique within the document context.

<div id="my-div1">This is my first element</div>
<div id="my-div2">This is my second element</div>
<svg id="my-div3">This is my third element</svg>

Passed Example 3

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Two of the id attributes are the same (my-elt), but they are in different trees (the first one in the document tree and the second in the shadow tree).

<div id="my-elt"></div>
<div id="host"></div>
<script>
	var host = document.getElementById('host')
	var shadow = host.attachShadow({ mode: 'open' })
	shadow.innerHTML = '<b id="my-elt"></b>'
</script>

Passed Example 4

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Both id attributes are the same (my-elt), but they are in different document trees because the iframe is creating a new one.

<div id="my-elt"></div>
<iframe title="Empty frame" srcdoc="<span id='my-elt'></span>"></iframe>

Failed

Failed Example 1

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The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field.

<div id="label">Name</div>
<div id="label">City</div>

<input aria-labelledby="label" type="text" name="city" />

Failed Example 2

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The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field.

<div id="label">Name</div>
<svg id="label">
	<text x="0" y="15">City</text>
</svg>

<input aria-labelledby="label" type="text" name="city" />

Failed Example 3

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The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field. This rule still considers elements that are neither included in the accessibility tree nor visible.

<span id="label" style="display: none;">Name</span>
<span id="label">City</span>

<input aria-labelledby="label" type="text" name="city" />

Inapplicable

Inapplicable Example 1

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There is no id attribute in this document.

<div>This is my first element</div>

Inapplicable Example 2

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The xml:id attribute is not considered by this rule.

<div xml:id="my-div">This is my first element</div>

Inapplicable Example 3

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These id attributes have an empty value.

<span id="">Hello</span> <span id="">world!</span>

Glossary

Focusable

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

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:

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.

Namespaced Element

An element with a specific namespaceURI value from HTML namespaces. For example an “SVG element” is any element with the “SVG namespace”, which is http://www.w3.org/2000/svg.

Namespaced elements are not limited to elements described in a specification. They also include custom elements. Elements such as a and title have a different namespace depending on where they are used. For example a title in an HTML page usually has the HTML namespace. When used in an svg element, a title element has the SVG namespace instead.

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:

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.

Programmatic Label

Element L is a programmatic label of target element T if either:

For more details, see examples of programmatic label.

Note: a given element may have more than one programmatic label.

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:

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

Note: The HTML standard suggests rendering elements with the hidden attribute with a CSS rule that applies the value none to the CSS property display of the element. Although the suggestion is not normative, known user agents render it according to the suggestion (unless the content specifies another CSS rule that sets the value of the display property). If a user agent does not follow the suggestion, this definition may produce incorrect results for this user agent.

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.

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 Consistency Complete Report
Alfa Consistent Yes View Report
Axe-core Consistent Yes View Report
QualWeb Consistent Yes View Report
SortSite Consistent Yes View Report

Changelog

This is the first version of this ACT rule.

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This is an unpublished draft preview that might include content that is not yet approved. The published website is at w3.org/WAI/.