Blog
Notes from building the validator
Specifics, not listicles. Everything here comes out of rules we had to write, edge cases that broke them, or disagreements between tools we had to resolve.
- SEO7 min read
Why does Google say "Discovered - currently not indexed"?
Google found the URL and decided not to spend a crawl on it. Almost always because nothing on your site links to it - a sitemap is a hint, and an internal link is the vote.
Read - SEO6 min read
What is a soft 404, and why does it cost you rankings?
A page that says "not found" in the body and 200 OK in the status line. Google trusts the status line, indexes the empty page, and spends your crawl budget rediscovering it.
Read - Performance7 min read
Why is my INP failing when Lighthouse says the page is fast?
Because Lighthouse cannot measure INP. It is a field metric that requires a real person clicking something, and the lab score you are reading is an estimate of a different number.
Read - SEO6 min read
Why does Search Console say "Duplicate without user-selected canonical"?
Google found several URLs serving the same content, you did not tell it which one counts, so it picked one itself - and it does not have to pick the one you would have.
Read - Security6 min read
Do security headers affect SEO?
Not directly - HTTPS is the only security signal Google has confirmed as a ranking factor. The indirect effects are real, and one misconfigured header can remove a page from the index entirely.
Read - AI Visibility9 min read
What is AI visibility?
Whether ChatGPT, Claude, Gemini and Perplexity can reach your site, quote it accurately, and act on it. It is three separate capabilities, and a site can pass one while failing the others.
Read - AI Visibility7 min read
How Googlebot differs from GPTBot
They crawl for different reasons, obey different rules, and reward different things. Optimising for one does not optimise for the other, and the settings that control them are entirely separate.
Read - AI Visibility6 min read
What is llms.txt, and should you have one?
A proposed file that points language models at your best content in plain Markdown. It is not a standard, it is not robots.txt, and it does not control access — here is what it does and does not do.
Read - AI Visibility8 min read
AEO vs SEO vs GEO: what the terms actually mean
Three acronyms for overlapping but genuinely different goals — ranking in a list, being quoted in an answer, and being represented in a generated response. Here is where each one applies.
Read - AI Visibility8 min read
What is generative engine optimization (GEO)?
Making your content likely to be retrieved, understood and represented accurately by generative AI. It starts with permission, not with writing — and most sites fail at the first step without knowing.
Read - AI Visibility7 min read
How to get cited by ChatGPT and other AI assistants
Citation depends on three things you control: whether the assistant may fetch you, whether your answer can be cleanly bounded, and whether there is anything to attribute it to.
Read - AI Visibility7 min read
Which structured data actually helps answer engines?
Four types carry most of the weight for AI and answer engines, and a long tail of speculative markup carries none. Here is what to add, in what order, and what to stop adding.
Read - HTML7 min read
Free HTML validator: which errors actually matter
A validator will hand you 200 errors. Perhaps six of them affect a real visitor. Here is how to tell which, and why the rest are still worth clearing eventually.
Read - CSS6 min read
Free CSS validator: why one typo removes styles you never touched
CSS fails silently and generously. A single malformed declaration causes the browser to discard the rest of its block, which is why the symptom is usually 'this style does nothing'.
Read - Structured Data7 min read
Free schema markup validator: valid is not the same as correct
Most structured data problems are not syntax errors. They are markup that parses perfectly and describes a page that does not exist.
Read - Structured Data6 min read
Free rich results test: eligibility is not a guarantee
Valid markup makes you eligible for a rich result. Four other things decide whether one appears, and only some of them are yours to control.
Read - Trust6 min read
Free website trust checker: what makes a site look legitimate
Visitors decide whether to trust a site in seconds, using a handful of signals. Most are trivial to add, and their absence is what makes a real business look like a template.
Read - AI Visibility7 min read
Free AI visibility checker: three capabilities, one scan
Whether AI assistants can reach your site, quote it accurately and operate it are three separate questions. Most sites pass one and fail the others, with no symptom either way.
Read - SEO6 min read
Free broken link checker: which 404s actually cost you
Not every reported failure is a broken link. Bot protection, rate limits and redirect chains all look like breakage, and fixing them wastes the time the real 404s deserve.
Read - SEO7 min read
Free SEO checker: the on-page issues that actually move rankings
On-page SEO is a small number of things done consistently. Here is the short list that accounts for most of what an audit finds, in the order worth fixing it.
Read - Accessibility7 min read
Free accessibility checker: what automation catches, and what it cannot
Automated testing finds roughly a third of WCAG failures. Knowing which third — and which two thirds still need a human — is the difference between compliance and a clean report.
Read - Security7 min read
Free security headers checker: what each header actually stops
Six headers, each defending against a specific attack. Here is what each one prevents, in the order worth adding them, and which one to deploy carefully.
Read - Performance7 min read
Free Core Web Vitals checker: lab numbers versus what users get
A lab measurement and your real users' experience are different numbers, and only one of them affects ranking. Here is how to read both.
Read - AI Agents6 min read
Free AI agent checker: can software complete a task on your site?
Agents book, buy and fill in forms on people's behalf. They do not see your page — they read the accessibility tree, which is why most sites are a dead end for them.
Read - HTML6 min read
Free W3C validator alternative: markup and CSS conformance in one pass
Conformance matters not because a badge says so, but because browsers recover from broken markup by guessing — and they do not all guess the same way.
Read - AI Visibility7 min read
Free answer engine optimization checker: is your content quotable?
Being crawlable is not being quotable. An engine has to bound your answer and attribute it, and most pages make both impossible without realising.
Read - AI Visibility7 min read
Free AI crawler checker: what your robots.txt actually permits
Reading robots.txt top to bottom gives the wrong answer surprisingly often. Group specificity and Allow precedence decide what a crawler may fetch — not the order of the lines.
Read - Security6 min read
Free SSL checker: expiry is only the failure everyone remembers
A certificate can be valid, current and still broken for a share of your visitors. Chain problems and protocol configuration fail selectively, which makes them hard to notice.
Read - SEO6 min read
Free robots.txt checker: the file that is easiest to get wrong
One line in robots.txt can remove a site from search entirely, and the resolution rules mean reading the file rarely tells you what it does.
Read - SEO6 min read
Free meta tag checker: the tags that build your search result
Titles and descriptions are what a result is assembled from, and duplicates across templated pages are the most common serious on-page problem there is.
Read - SEO6 min read
Free Open Graph checker: why your link preview is blank
A preview is generated at share time by a scraper that does not run JavaScript and gives up quickly. Almost every broken preview comes down to one of four causes.
Read - SEO6 min read
Free image SEO checker: the largest thing on your page, least examined
Images are usually most of a page's weight and most of its accessibility failures. Five changes fix ranking, load time and screen reader usability at once.
Read - SEO6 min read
Free mobile-friendly test: what to use now Google retired theirs
Google removed its mobile-friendly test in 2023. The requirements did not change, and mobile-first indexing means the mobile version is the one being ranked.
Read - SEO8 min read
Free website checker: what a full audit should actually cover
A complete audit spans indexability, accessibility, security, structured data, performance and AI visibility. Here is what each contributes, and the order worth fixing them in.
Read - SEO6 min read
Why is my robots.txt Disallow being ignored?
Almost always group specificity. A crawler obeys exactly one group - the most specific one that matches its name - and ignores every other rule in the file.
Read - SEO5 min read
Why does my link preview show no image?
Almost always a relative og:image URL. Scrapers fetch your image cold from a different network with no page context, so the URL has to be absolute.
Read - Accessibility6 min read
What can automated accessibility testing actually catch?
Roughly a third of WCAG issues. Automated tools find machine-checkable failures reliably and are structurally unable to judge whether alt text is meaningful.
Read - Performance5 min read
Google retired the Mobile-Friendly Test — what now?
It was removed in December 2023. Mobile did not stop mattering: Google indexes the mobile rendering of your page by default, so it is the version that ranks.
Read - SEO5 min read
Why do link checkers report broken links that work fine?
Because a large share of the web answers automated clients with 403 or 429 while serving humans normally. That is bot protection, not a broken link.
Read - AI Visibility7 min read
What is answer engine optimization?
AEO is making content usable by systems that answer directly instead of returning links. The mechanics differ: structure and attributability matter more than keywords.
Read - HTML6 min read
Why does my HTML pass in the browser but fail validation?
Because browsers are required to repair broken markup rather than reject it. The page renders, and the DOM you get is not the one you wrote.
Read - CSS6 min read
Why did my CSS declaration silently stop working?
CSS has no error reporting. A browser drops any declaration it cannot parse and continues, so a single typo produces a layout bug with no error anywhere.
Read - Structured Data7 min read
Why is my structured data valid but showing no rich results?
Valid and eligible are different tests. Each rich result type has required properties, and markup can pass every syntax check while qualifying for nothing.
Read - Trust7 min read
How can you tell if a website is legitimate?
No single signal proves it. Legitimate sites accumulate verifiable details a fraudulent one has no reason to produce - and the absence of several together is the real signal.
Read - AI Agents7 min read
Can AI agents actually use your website?
Agents drive real browsers and read the accessibility tree. A site can be perfectly crawlable and completely unusable by one - and most sites have never been tested.
Read - Security5 min read
What actually happens when an SSL certificate expires?
Every visitor gets a full-page interstitial they must click through. The site is not slow or degraded - it is unreachable to anyone who trusts their browser.
Read - AI Visibility6 min read
Which AI crawlers can actually read your site?
GPTBot, OAI-SearchBot, ClaudeBot and Google-Extended are separate tokens with separate consequences. Blocking one does not block the others, and allowing Googlebot does not allow any of them.
Read - Performance5 min read
Why your Lighthouse score disagrees with everyone else's
Two tools measuring the same page routinely disagree by thirty points. Almost always it is throttling, cold cache, or lab versus field data — not a bug.
Read - Structured Data7 min read
Structured data that actually earns rich results
Valid JSON-LD is not the same as eligible JSON-LD. The gap is usually required properties, mismatched content, or markup describing something the page does not show.
Read
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