HTML page lang and xml:lang attributes have matching values

Description

This rule checks that both lang and xml:lang attributes on the root element of a non-embedded HTML page, have the same primary language subtag.

Applicability

This rule applies to any document element if it is an html element for which all the following are true:

is in a top-level browsing context; and
has a node document with a content type of text/html; and
has a lang attribute with a known primary language tag; and
has a non-empty xml:lang attribute.

Expectation

For each test target, the values of the primary language subtags, if any exist, for the lang and xml:lang attributes are the same.

Assumptions

The language of the page can be set by other methods than the lang attribute, for example using HTTP headers or the meta element. These methods are not supported by all assistive technologies. This rule assumes that these other methods are insufficient to satisfying Success Criterion 3.1.1: Language of Page.
This rule assumes that user agents and assistive technologies can programmatically determine known primary language tags even if these do not conform to the RFC 5646 syntax.
This rule assumes that only known primary language tags are enough to satisfy Success Criterion 3.1.1 Language of Page; this notably excludes grandfathered tags and ISO 639.2 three-letters codes, both having poor support in assistive technologies.
The rule assumes that having lang and xml:lang attributes with matching primary language subtags but non-matching language tags overall, will not cause accessibility issues. This is not necessarily the case for all languages. One notable case is the language tags for Cantonese (zh-yue) and Mandarin (zh-cmn) where the primary language subtags match, but the extended language subtags don’t. Such a case would not fail this rule, but could lead to accessibility issues.

Accessibility Support

Since most assistive technologies will consistently use lang over xml:lang when both are used, violation of this rule may not necessarily be a violation of WCAG 2. Only when there are inconsistencies between assistive technologies as to which attribute is used to determine the language does this lead to a violation of SC 3.1.1.

Background

This rule is only applicable to non-embedded HTML pages. HTML pages embedded into other documents, such as through iframe or object elements are not applicable because they are not web pages according to the definition in WCAG.

Bibliography

Accessibility Requirements Mapping

3.1.1 Language of Page (Level A)
- Learn more about 3.1.1 Language of Page
- 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

Input Aspects

The following aspects are required in using this rule.

DOM Tree

Test Cases

Passed

Passed Example 1

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This html element has identical primary language subtags for its lang and xml:lang attributes.

<html lang="EN" xml:lang="en"></html>

Passed Example 2

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This html element has identical primary language subtags for its lang and xml:lang attributes. The extended language subtags also match.

<html lang="en-GB" xml:lang="en-GB"></html>

Passed Example 3

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This html element has identical primary language subtags for its lang and xml:lang attributes. The extended language subtags do not match, but this is not required by this rule.

<html lang="en-GB" xml:lang="en-US"></html>

Failed

Failed Example 1

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This html element has different primary language subtags for its lang and xml:lang attributes.

<html lang="fr" xml:lang="en"></html>

Failed Example 2

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This html element has different primary language subtags for its lang and xml:lang attributes. The extended language subtags do match, but this rules only focus on the primary language subtags.

<html lang="fr-CA" xml:lang="en-CA"></html>

Inapplicable

Inapplicable Example 1

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This rule does not apply to svg elements.

<svg xmlns="http://www.w3.org/2000/svg" lang="en" xml:lang="en"></svg>

Inapplicable Example 2

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This rule does not apply to svg elements, even inside an html element.

<html>
	<body>
		<svg lang="en"></svg>
	</body>
</html>

Inapplicable Example 3

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This rule does not apply to math elements.

<math xml:lang="en"></math>

Inapplicable Example 4

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This rule only applies to documents with a content type of text/html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html lang="en" xml:lang="en"></html>

Inapplicable Example 5

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This rule does not apply to html elements without an xml:lang attribute.

<html lang="en"></html>

Inapplicable Example 6

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This rule applies neither to html elements without an xml:lang attribute, nor to html in nested browsing context

<html lang="en">
	<iframe srcdoc="<html lang='en' xml:lang='en'></html>" />
</html>

Inapplicable Example 7

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This rule does not apply to html elements with an empty ("") xml:lang attribute.

<html lang="fr" xml:lang=""></html>

Glossary

Known Primary Language Tag

A language tag has a known primary language tag if its primary language subtag exists in the language subtag registry with a Type field whose field-body value is language.

A “language tag” is here to be understood as in the first paragraph of the RFC 5646 language tag syntax, i.e. a sequence of subtags separated by hyphens, where a subtag is any sequence of alphanumerical characters. Language tag that are not valid according to the stricter RFC 5646 syntax (and ABNF grammar) definition can still have a known primary language tag. User agents and assistive technologies are more lenient in what they accept. This definition is consistent with the behavior of the :lang() pseudo-selector as defined by Selectors Level 3.

As an example, de-hello would be an accepted way to indicate German in current user agents and assistive technologies, despite not being valid according to RFC 5646 grammar. It has a known primary language tag (namely, de).

As a consequence of this definition, however, grandfathered tags do not have a known primary language tag.

Subtags, notably the primary language subtag, are case insensitive. Comparison with the language subtag registry must be done in a case insensitive way.

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.

Rule Versions

Proposed version, 21 June 2022 (compare)
Latest version, 28 January 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 0.36.0	Automated tool	Consistent
Axe DevTools Pro 4.24.5	Semi-automated tool	Consistent
Axe-core 4.5.0-prerelease	Automated tool	Consistent
Equal Access Accessibility Checker 3.1.36	Automated tool	Consistent
QualWeb 3.0.0	Automated tool	Consistent
SortSite 6.45	Automated tool	Partial