ACT-Rules Community

Id attribute value is unique

  • Rule Type:atomic
  • Rule Id: 3ea0c8
  • Last modified: Jan 11, 2022
  • Accessibility Requirements Mapping:
    • 4.1.1 Parsing (Level A)
      • Learn More about 4.1.1 Parsing
      • 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.
    • H93: Ensuring that id attributes are unique on a Web page
      • Learn More about technique H93
      • Not required to conformance to any W3C accessibility recommendation.
      • Outcome mapping:
        • Any failed outcomes: technique is not satisfied.
        • All passed outcomes: technique is satisfied.
        • An inapplicable outcome: technique is satisfied.
  • Input Aspects:

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

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:

  • 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.

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:

  • 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.

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.

Useful Links

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. This section will not be included in the rule when it is published on the W3C website.

ToolConsistencyCompleteReport
AlfaconsistentYesView Report
axe-coreconsistentYesView Report
Equal Access Accessibility CheckerconsistentYesView Report
QualWebconsistentYesView Report
SortSiteconsistentYesView Report

Acknowledgments

Authors

Funding

  • WAI-Tools
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