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HTML5 Validator – Check Markup & Prettify Output

Validate HTML5 documents for structural issues, mismatched tags, and invalid attributes with optional prettified output and Markdown reports.

🟢 Runs locally · no uploads

HTML5 Validator

Validate HTML5 markup with tag hierarchy and deprecation warnings

htmlHTML Markup
0 chars•0 tag(s)•0 B
html

Enter HTML markup to validate

Or drag and drop an HTML file

Validation Result
task_alt

Validation result will appear here

Characters: 0Lines: 1Size: 0 B

lightbulbPopular Use Cases

bug_report
Website QA Testing

Validate HTML markup for accessibility, SEO, and cross-browser compatibility issues

mail
Email Template Validation

Check HTML email templates for compliance with email client rendering standards

edit_note
CMS Content Review

Validate user-generated HTML content before publishing to catch markup errors

accessibility
Accessibility Audits

Verify HTML5 semantic elements and ARIA attributes for WCAG compliance

scienceExample HTML

htmlValid HTML5
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Page Title</title>
  </head>
  <body>
    <h1>Hello World</h1>
  </body>
</html>
htmlSimple Section
<section>
  <h2>Introduction</h2>
  <p>This is a paragraph.</p>
</section>

keyboardKeyboard Shortcuts

keyboardShow shortcutsexpand_more
Ctrl+EnterValidate HTML
Ctrl+LClear all

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› About this tool · FAQ

Validate HTML5 markup for unclosed tags, structural issues, and malformed attributes with optional prettified output and Markdown reports.

Does this validator support HTML5?

Yes. It checks HTML5 documents for common structural issues like unclosed tags, mismatches, and invalid attributes.

Will this tool prettify my HTML?

Enable the prettify option to output formatted HTML while validating, making diffs easier to review.

Do you send my HTML to a server?

No. Validation and formatting happen entirely in your browser, keeping staging or production markup private.

Can I ignore warnings?

Yes. Toggle the warnings option to hide advisory messages and focus on blocking errors.

Does it detect accessibility issues?

It focuses on structural HTML validation. Pair it with accessibility testing tools for comprehensive reviews.