Technical SEO fails quietly. No error message appears when your canonical tags contradict your sitemap, when a firewall starts serving 403 to crawlers, or when a redesign ships every meta description as the same sentence. Things simply stop working, weeks pass, and traffic charts drift downward with no obvious cause. This checklist exists to catch those silent failures. Twenty five points, grouped into six areas, each with a concrete way to verify it. Work through it quarterly and after every major site change.

Crawling and Indexing

1. robots.txt allows what it should. Fetch yoursite.com/robots.txt and read every Disallow line out loud. Verify the file served publicly matches the file on your server, CDNs can rewrite it in transit. Full rules in our robots.txt guide.

2. The XML sitemap lists only clean URLs. Every listed URL should return 200, be canonical and be indexable. No redirects, no parameters, no thin auto generated pages. See the sitemap guide for what belongs and what does not.

3. lastmod dates are honest. If your sitemap stamps today on every URL every day, search engines ignore the field entirely.

4. Every page has exactly one valid canonical. Self referencing on clean URLs, pointing at the clean version on parameter variants, never at redirects or 404s. The canonical guide lists the failure modes.

5. Host variants collapse in one hop. The http, https, www and non www versions of any URL should reach the canonical form in a single 301. Test with curl and count the hops, chains are covered in our status code guide.

6. Deleted pages answer honestly. Removed content returns 404 or 410, not a friendly page with status 200, and not a blanket redirect to the homepage.

On Page Signals

7. Unique title on every page. Around 50 to 60 characters, the specific topic first, the brand last. Duplicated titles across pages are the fastest thin content signal you can send.

8. Unique meta description on every page. One or two sentences that would make a human click. Our meta tags guide covers the full head section.

9. One H1 per page, then a logical heading tree. H2 sections, H3 subsections, no skipped levels, no headings chosen for font size.

10. Open Graph and Twitter Card tags on every shareable page. Title, description, a 1200 by 630 image with an absolute URL. Broken previews cost clicks on every share, details in the Open Graph guide.

11. Structured data that matches the visible page. Valid JSON-LD for your organization, articles, products or local business, tested in a validator. Start with our structured data guide.

Performance

12. Core Web Vitals in the green. LCP under 2.5 seconds, INP under 200 milliseconds, CLS under 0.1, measured on real user field data, not just lab runs. The Core Web Vitals guide explains each metric and its levers.

13. Images in modern formats at honest sizes. AVIF or WebP with fallbacks, responsive srcset variants, dimensions declared, compression applied. The image optimization guide is the recipe.

14. Lazy loading below the fold only. The hero image loads eagerly with fetchpriority high, everything below the viewport gets loading lazy. Lazy loading the LCP image is a self inflicted wound.

15. Text compression and caching enabled. Brotli or gzip on HTML, CSS and JavaScript, long cache lifetimes on static assets with versioned filenames.

16. Third party scripts on a leash. Every external script earns its place or leaves, and whatever survives loads deferred or after first interaction. Tag managers accumulate corpses, audit them yearly.

Mobile and UX

17. Real responsive behavior. The viewport meta tag present, layouts tested at 360 pixels wide, tap targets big enough for thumbs.

18. No intrusive interstitials. Popups that cover content on entry hurt both rankings and humans. Banners that leave the content readable are fine.

19. Accessibility basics in place. Alt text on meaningful images, labels on form fields, visible focus states, sufficient color contrast. Accessibility overlaps heavily with machine readability, as our accessibility guide shows.

Security and Trust

20. HTTPS everywhere with no mixed content. Valid certificate, every subresource loaded over https, http redirecting permanently. Background in the HTTPS guide.

21. Security headers configured. HSTS, X-Content-Type-Options, a sane frame policy and a Content Security Policy appropriate to your stack, all covered in the security headers guide.

22. Contact, about and policy pages exist and are findable. Trust evaluators, human and algorithmic, look for a real organization behind the site.

AI Readiness

23. AI crawlers get a deliberate answer. You decided which of the 18 AI bots to allow, robots.txt reflects the decision, and a live fetch test confirms no CDN or firewall quietly overrides it with 403s.

24. Content exists in raw HTML. Your main text, headings and meta tags are present before JavaScript runs. View source, not inspect element, tells the truth, and the JavaScript visibility guide shows the fixes.

25. An llms.txt file describes your site. Fifteen minutes to create, near zero cost, read by real retrieval systems today. Format and examples in the llms.txt guide.

Run the Whole Checklist in Ten Seconds

Every point above is checkable by hand, and the by hand version is exactly how you should understand each one the first time. After that, automation wins: the CheckMy.site scanner tests all of these signals and more, 158 checks across crawlability, on page SEO, performance, security, accessibility and AI readiness, in a single free scan with a prioritized fix list. Print this checklist, scan your site, and close the gaps one by one. Technical SEO is not glamorous, it is the plumbing, and plumbing is what decides whether everything else you build actually flows.