How to fix html has lang on Headless / Custom (I build in code)

Steps for Headless / Custom (I build in code)

1. Locate the file that renders the root HTML document — typically index.html, _document.js, app.html, layout.html, or your framework's equivalent.
2. Find the `<html>` tag and add a `lang` attribute with the correct BCP 47 language code, e.g., `<html lang="en">`.
3. If you serve multiple languages, set the lang attribute dynamically based on the current locale/route.
4. Remove any `xml:lang` attribute if present (it is not valid in HTML5).
5. Deploy and verify via View Source or browser DevTools.

What is html has lang?

Every HTML page should declare its primary language in the opening `<html>` tag using a `lang` attribute — for example, `<html lang="en">` for English. This single attribute tells browsers, screen readers, and search-engine crawlers what human language the page content is written in. WCAG Success Criterion 3.1.1 ("Language of Page") requires it. A common mistake is using the XML version (`xml:lang`) instead of the standard HTML `lang` attribute — only `lang` is valid in HTML documents.

Screen readers use the `lang` attribute to select the correct pronunciation engine and voice profile; without it, a visually-impaired customer may hear your entire site read aloud in the wrong language, making it effectively unusable. This is a Level A WCAG violation — the lowest bar for accessibility compliance — and is frequently cited in accessibility audits and legal complaints under the ADA, the European Accessibility Act, and equivalent laws worldwide, exposing you to real legal and financial risk. Search engines also use the declared language to serve your pages to the right audiences, so a missing or wrong `lang` value can quietly undermine your international SEO. Fixing it takes less than five minutes and has zero negative side effects.

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