Individual skill

html-xml-lang-mismatch

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 needed and `xml:lang` is not required. Check when auditing documents served as application/xhtml+xml or documents that include both attributes.

skills/html-xml-lang-mismatch/SKILL.md

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Matches the observed repository HEAD: Yes
Scanner rules are current: No
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Observed behavior: Not checked yet
Last checked: Nov 2, 2026

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Repository stars (not skill ratings): 74,679
Repository last updated: Oct 31, 2026
License: MIT

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frontend Repository topic
front-end-development Repository topic
front-end-developer-tool Repository topic
guidelines Repository topic
checklist Repository topic
reference Repository topic
web-development Repository topic
html Repository topic
css Repository topic
javascript Repository topic
engineering Derived category

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SKILL.mdcore · 3,202 bytes
references/rule.mddoc · 3,660 bytes

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# Match lang and xml:lang attributes

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.

## Quick Reference

- If both `lang` and `xml:lang` are present on `<html>`, their values must be identical
- `xml:lang` is only needed for XHTML or polyglot documents (valid as both HTML and XML)
- For standard HTML5 documents, only `lang` is required — `xml:lang` is redundant
- The primary language tag must be a valid BCP 47 language code (e.g., `en`, `en-US`, `fr`, `zh-Hant`)
- Relates to WCAG 2.1 SC 3.1.1 (Language of Page)

## Check