Use when applies to XHTML documents and polyglot HTML documents that must parse correctly as both HTML5 and XML. For modern HTML5-only documents, only `lang` is
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Screen readers use the lang attribute to select the correct voice profile and pronunciation engine. If lang and xml:lang disagree, XML-based processors (including some EPUB readers and older assistive technologies) may use xml:lang and get a different language than intended — causing text to be read aloud in the wrong language. For example, French text read with an English voice profile produces unintelligible output for French-speaking blind users.
lang and xml:lang are present on <html>, their values must be identicalxml:lang is only needed for XHTML or polyglot documents (valid as both HTML and XML)lang is required — xml:lang is redundanten, en-US, fr, zh-Hant)Inspect the <html> element for both lang and xml:lang attributes. If both are present, verify their values are identical (including subtags — en vs en-US is a mismatch). Also verify that the value is a valid BCP 47 language tag. Flag if the values differ in any way.
If lang and xml:lang are present but differ: update both to the same valid BCP 47 language code matching the primary language of the document content. If this is a standard HTML5 document (served as text/html), remove xml:lang — it is unnecessary. If this is XHTML or a polyglot document, keep both and ensure they are identical.
WCAG 2.1 SC 3.1.1 requires that the default human language of each web page can be programmatically determined. The lang attribute on <html> serves this purpose for HTML parsers. The xml:lang attribute serves the same purpose for XML parsers. When a document must be valid as both HTML and XML (polyglot), both attributes must be present and identical. A mismatch means the document claims different languages depending on which parser reads it, which can cause screen readers, translation tools, and spell checkers to use wrong language rules.
Review the rendered markup and interactive states that affect Match lang and xml:lang attributes. Flag exact elements, roles, labels, focus behavior, or keyboard interactions that violate the rule, and note how to verify the fix with browser accessibility tooling or assistive tech.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/en/rules/accessibility/html-xml-lang-mismatch