Why it matters
Screen readers, Bing, translation and agents use lang to process the text; an invalid tag is ignored.
How to fix it
Set a valid BCP 47 tag on the root element (en, en-GB, pt-BR).
Example
// app/layout.tsx
export default function RootLayout({ children }: { children: React.ReactNode }) {
return <html lang="en"><body>{children}</body></html>
}References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
html-too-largeUncompressed HTML is over Googlebot's 2 MB fetch limitrsc-payload-ratioInline RSC flight payload dominates the HTMLnot-httpsPage served over plain HTTPorigin-redirectshttp:// or www/apex variant doesn't redirect to the canonical origin in one hopviewportViewport meta missing, not device-width, or blocks zoomcharsetCharacter encoding not declared earlytitle-missingNo single non-empty <title> in <head>title-lengthTitle length outside 10–60 characters, or keyword-stuffed