Standards-compliant markup
Enter a URL or paste HTML and it's checked by the W3C Nu HTML Checker — the same engine behind validator.w3.org. Every error and warning is listed with its line and column and the offending code extract.
Invalid markup rarely hurts rankings directly, but broken nesting, duplicate IDs, unclosed tags and missing attributes can break layouts, confuse screen readers and make pages harder for search engines to parse reliably.
How to validate HTML
- 1
Enter a URL or paste HTML
Full documents or fragments.
- 2
Run the validator
A few seconds.
- 3
Fix errors first
Then review warnings.
Key features
Official W3C engine
The Nu HTML Checker.
URL or pasted code
Two ways to check.
Line numbers and extracts
Find problems fast.
Errors vs warnings
Prioritize what matters.
Common uses
- Debugging layouts: Find unclosed or misnested tags.
- Accessibility work: Catch missing alt and label issues.
- Email and CMS templates: Clean up generated HTML.
Tips and good to know
- Fix the first error before worrying about later ones; one unclosed tag can cause a cascade of messages.
- Tidy the code for reading with the HTML Formatter.
Frequently asked questions
Does HTML validation affect SEO?
Not directly — Google handles imperfect HTML — but serious errors can hide content or break structured data and layouts.
Which HTML version is checked?
The living HTML standard (HTML5), as maintained by WHATWG and W3C.
Why do I see errors from third-party scripts?
Validation checks the full page source, including embedded widgets and ad code you may not control.