I wrote a commit message that says SEO.tsx "sets documentElement.lang" and "emits hreflang links". I opened a Spanish post in the browser, looked at the document, and there it was: documentElement.lang === "es", and three <link rel="alternate" hreflang> tags for es, en, and x-default. Right language, right pairs, right default.
The commit message was accurate. The site was also serving <html lang="en"> on every Spanish post, with zero hreflang annotations.
Both of those sentences are true. They are true of different surfaces.
The annotation that was there and wasn't
This site is a React 19 SPA on Vercel with a build-time prerender step. The prerender writes the <head> into the static HTML so that crawlers get titles and meta tags without executing anything. SEO.tsx is a React component. It runs in the browser, after React mounts, and it does exactly what its commit message claims.
