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Free Online HTML Validator

Check your HTML structure online. Detect missing tags, structural errors, and common HTML problems instantly in your browser.

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HTML validation checks whether your HTML document is well-structured and follows the rules of the HTML specification. Unlike strict XML, HTML parsers are highly tolerant of errors and will often render something even when the markup is technically invalid. However, invalid HTML can behave unexpectedly across browsers, cause accessibility issues, and break CSS selectors.

JSON Viewer Pro's HTML validator uses the browser's native HTML5 parser to check your HTML. It identifies structural problems such as unclosed tags, incorrect nesting, and missing required elements. Common issues like missing DOCTYPE declarations, invalid attribute values, and deprecated elements are flagged with descriptive messages.

The validator is useful for checking HTML from templates, CMS exports, web scrapers, and email clients, which often produce non-standard or legacy markup. All validation runs locally in your browser — your HTML is never sent to any external service.

Key Features

  • ✓ Check HTML structure and nesting
  • ✓ Detect unclosed or mismatched tags
  • ✓ Identify structural HTML errors
  • ✓ Real-time validation as you type
  • ✓ Works with HTML fragments and full pages
  • ✓ Check HTML5 document structure
  • ✓ No data sent to any server
  • ✓ Free and no login required

Common Use Cases

  • → Validate HTML templates
  • → Check CMS-generated HTML
  • → Validate email HTML for clients
  • → Check HTML from web scrapers
  • → Validate HTML before deploying
  • → Check HTML from third-party tools
  • → Validate static site HTML output
  • → Check HTML snippets for bugs

Frequently Asked Questions

What does HTML validation check?

Structure, tag matching, nesting correctness, and common markup errors. It uses the browser's native HTML5 parser.

Is this the same as the W3C validator?

This is a fast browser-based structural check. For full W3C compliance checking, the official W3C Markup Validation Service provides more detailed reports.

Does it detect accessibility issues?

Not currently. The validator focuses on structural HTML errors. Accessibility checking (ARIA, alt text, etc.) is on the roadmap.

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