On-page SEO audit of one URL: title and meta description length, canonical, robots meta and X-Robots-Tag, robots.txt allow for user-agent *, sitemap.xml with URL count, lang, hreflang, h1-h3 outline, word count, images without alt, internal/external links, Open Graph, Twitter card, JSON-LD schema.org types, favicon, viewport, mixed content; each check pass/warn/fail with a fix and a 0-100 score.