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HTML Validator

Validate

Validate HTML against W3C standards. Catch errors, missing attributes and bad nesting.

W3C ✓
Input
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✔

Paste HTML then click Validate.

Checks against the W3C Nu HTML Checker.

✨ AI Code Explanation
Mode

What is an HTML Validator?

An HTML validator checks your markup against the official W3C HTML standard. It catches missing attributes, unclosed tags, bad nesting, deprecated elements, and other issues that can break rendering or accessibility — before they reach production.

How to Validate HTML Online

  1. Paste your HTML into the Input pane, or click Upload to load a file.
  2. Click Validate — results appear in the right panel within seconds.
  3. Each issue shows its type (error / warning / info), line and column, message, and the offending code extract.
  4. Click Explain with AI to get a plain-English breakdown of what each error means and how to fix it.

Errors, Warnings, and Info

  • Error — invalid markup. Browsers may misrender, skip, or recover unexpectedly from this element.
  • Warning — technically valid but worth reviewing (e.g. missing lang attribute, obsolete attributes).
  • Info — informational note about parsing behaviour. No action typically required.

Common HTML Validation Errors

  • Missing alt on <img> — required for accessibility and screen readers.
  • Duplicate id attributes — each id must be unique per page.
  • Unclosed tags — a missing </div> can collapse entire layout sections.
  • Incorrect nesting — e.g. <p><div>…</div></p> is invalid; block elements cannot be nested inside <p>.
  • Missing DOCTYPE — triggers quirks mode in older browsers, causing layout differences.
  • Deprecated elements — <center>, <font>, <b> without semantic intent are flagged.

Why Validate HTML?

Valid HTML renders consistently across all browsers, improves SEO (search engines parse semantic markup better), and ensures your page is accessible to screen readers and assistive technologies. The W3C validator is the industry-standard reference used by web professionals worldwide.

Related Tools

Is My Code Sent to a Server?

Validation sends your HTML to the public W3C Nu HTML Checker API. No data is stored — the service processes and immediately discards it. All other tools on this site run entirely in your browser.