Implement html-element-lang-matches rule (off6ek)

[koddsson]

ACT Rule

• ACT Rule ID: off6ek
• Rule Name: HTML element language subtag matches language
• WCAG SC: 3.1.2 Language of Parts (Level AA)

What it checks

This rule checks that the lang attribute value on HTML elements matches the actual language of the text content within those elements.

Feasibility

DOM-only feasible. The check involves validating lang subtag format and consistency of lang attribute values on elements.

Notes

• Related to issue 35 W3C ACT rules have no corresponding scanner rule #350 (ACT rules triage)
• The existing valid-lang rule checks lang attribute validity; this rule checks that lang matches actual content
• May require a language detection heuristic or could focus on structural checks

[koddsson]

After reviewing the ACT test cases, this rule requires language detection — it checks that the lang attribute on an element matches the actual language of the text content (e.g., Dutch text marked as lang="fr" is a failure). This is not feasible for a DOM-only scanner without integrating a language detection library. Marking as out of scope for now.