All checks / html-xml-lang-mismatch
Ensure that HTML elements with both valid lang and xml:lang attributes agree on the base language of the page
The European Accessibility Act requires covered digital services to meet the harmonised standard EN 301 549. Clause 9 of that standard adopts the WCAG 2.1 Level AA success criteria directly, so a failure of SC 3.1.1 is a failure of EN 9.3.1.1 — which is the reference a supervising authority or a complainant will cite, not the WCAG number on its own.
The html-xml-lang-mismatch check is machine-decidable: where it reports a failure,
the failure is real. It cannot confirm the opposite. Passing this check does not mean
you satisfy EN 9.3.1.1, because parts of that criterion depend on
human judgement about meaning and context.
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