HTML Validator (W3C)

Validate the HTML of any page with the official W3C checker and list every error and warning with its line.

How do I validate the HTML of my website?

Enter the page URL; our server downloads the HTML and sends it to the official W3C Nu HTML Checker, then lists every error and warning with the line number, the message and the offending code.

Errors worth fixing first

Unclosed or wrongly nested elements, which can break layouts.
Duplicate id attributes, which break labels, anchors and scripts.
Missing alt on images and invalid ARIA attributes (accessibility).
Stray end tags inside <head>, which can stop Google reading meta tags.

Frequently asked questions

Does valid HTML improve rankings?

Not directly, but serious errors can break rendering, accessibility and structured data.

Can I validate HTML I paste?

This tool checks pages by URL; the W3C site accepts direct input.

Is it free?

Yes.