HTML Checker Online: Find Errors Crawlers Trip On
An online HTML checker validates a page against the HTML standard and lists the errors. Enter a URL: we run it through the W3C Nu validator and put the errors that affect how crawlers read the page first — invalid elements in the head, broken nesting, duplicate IDs — ahead of the cosmetic ones.
Validation is done by the W3C Nu HTML Checker, the same engine behind validator.w3.org. We sort its findings by SEO impact. Free, no signup. Updated Oct 24, 2026 by the AEOeye team.
Does valid HTML matter for SEO?
Mostly no — with one important exception. Google says it tries to understand HTML even when it's invalid, so a page full of minor errors can rank fine. The exception is the head: if Google meets an invalid element in the head, it ignores everything after it.
That makes a misplaced <img>, <div> or <iframe> near the top of the head genuinely dangerous: your canonical, robots tag or hreflang can sit right below it, invisible to Google.
Which errors should I fix first?
We sort errors into SEO-relevant and other, so the dangerous ones come first:
- Elements not allowed in the head — the problem above.
- Stray or unclosed tags around the head and body — parsers may close the head early.
- Duplicate IDs — break anchors, labels and scripts.
- Broken nesting in links and headings — can garble anchor text and outlines.
Attribute warnings and obsolete markup come last; fix them when convenient.
Why use the W3C Nu validator?
It's the reference implementation of the HTML standard, maintained by the W3C and used by validator.w3.org. Running it through this page adds the URL fetch and the SEO-first sorting, and keeps the full list one click away.
What does this look like on real sites?
What matters on real sites is the head, not perfection. In our October 2026 analysis of 315 big-brand homepages (dataset), we could read a canonical tag on 94.3% and a meta description on 97.8%. When one of those tags is in your source but a checker can't find it, a head error above it is the first suspect.
If the validator flags a head problem, confirm what Google actually sees with the meta tag checker and the raw markup in view page source.
One page checked. What about the rest of your site?
The free SEO audit crawls your site and scores ~90 checks across 7 areas — with the evidence and the fix for each problem.
FAQ
Is HTML validation a ranking factor?
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No. Google tries to understand invalid HTML, and validity itself is not a ranking signal. Errors matter only when they stop Google from reading metadata, especially inside the head.
Why does the validator report so many errors?
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Modern sites include frameworks, third-party scripts and attributes that strict validation flags. Most are harmless. Focus on the errors we mark as SEO-relevant.
Can I validate a page behind a login?
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No. We fetch public URLs only. For private pages, copy the HTML from your browser and paste it into validator.w3.org/nu directly.
Sources
Related free tools
Meta Tag Checker
Free meta tag checker: see every tag that matters on a URL — title, description, robots, canonical, viewport, Open Graph, X cards — graded with fixes.
View Page Source
View page source of any URL online: HTML with line numbers and search, minified pages formatted, plus title, meta, canonical and JSON-LD found by line.
On-Page SEO Checker
Free on-page SEO checker: grade one URL on 25 checks — title, meta, headings, content, images, internal links, schema, indexability — each with a fix.
Schema Validator
Free schema validator: parse every JSON-LD block on a URL and flag syntax errors and missing required properties per Google's structured data docs.