--- obsoleteMessage: | This failure relates to 4.1.1: Parsing, which was removed as of WCAG 2.2. obsoleteSince: 22 ---

# Failure of Success Criterion 4.1.1 due to duplicate values of type ID

ID: F77

Technology: failures

Type: Failure

## When to Use

HTML, and any XML-based markup languages including SVG

## Description

This describes a failure condition where duplicate `id` errors are known to cause problems for assistive technologies when they are trying to interact with content. Duplicate values of type `id` can be problematic for user agents that rely on this attribute to accurately convey relationships between different parts of content to users. For example, a screen reader may use `id` values to identify the applicable header content for a data cell within a data table, or an input control to which a given label applies. If these values are not unique, the screen reader will be unable to programmatically determine which headers are associated with the data cell or which control is associated with which label or name.

Checking that `id` attribute values are unique within a document can be done by validating the document against its specification, because the specification defines which attributes contain document-wide unique identifiers.

In most markup languages, `id` values are attribute values, for example in HTML and SVG.

XML documents that use only the `xml:id` attribute as an `id` attribute, parsing the XML document with a validating parser that supports the xml:id specification is sufficient.

## Examples

An author uses an online validation service to check that all `id` attribute values are unique.

A developer utilizes features in their authoring tool to ensure that `id` attribute values are unique.

## Tests

### Procedure

1. Check that all values of type `id` are unique in the Web page

### Expected Results

- If check #1 is false, then this failure condition applies and the content fails the Success Criterion.

## Related Techniques

- G108
- H75

## Resources

- HTML: id attribute
- xml:id Version 1.0 - W3C Recommendation 9 September 2005.
- Extensible Markup Language (XML) 1.0 (Fourth Edition): Validity constraint: ID