Tools / HTML Tag Checker

HTML Tag Checker

Validate HTML online — detect unclosed tags, extra closing tags, mismatched nesting, deprecated tags, obsolete attributes, duplicate IDs and accessibility issues. Auto-fix, tag tree, sandboxed preview. 100% private, no signup.

Private Everything runs in your browser — nothing is uploaded
Upload HTML File
HTML Source
0 tags 0 lines 0 bytes
Checks
Fix

Auto-fix only closes unclosed tags and removes extra closing tags. It does not modify attributes or content.

Validation Report Paste HTML to check
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Waiting for HTML
0
Errors
0
Warnings
0
Info
0
Max depth

Paste HTML to see issues

What is checked
  • Unclosed tags — tags opened but never closed (e.g. <div> without </div>).
  • Extra closing tags — closing tags with no matching opening tag.
  • Mismatched nesting — overlapping tags like <b><i></b></i>.
  • Deprecated tags — center, font, marquee, big, strike, tt and others.
  • Obsolete attributes — align, bgcolor, border, valign on modern elements.
  • Duplicate IDs — the same id used more than once (invalid HTML).
  • Accessibility — missing alt on <img>, missing lang on <html>, empty href.
  • Inline event handlers — onclick, onload, etc. — a security/SEO warning.
Runs 100% in your browser. Your HTML never leaves your device.
Tip: The preview renders in a sandboxed iframe with scripts disabled — safe even for untrusted HTML.

What is the HTML Tag Checker?

Validate HTML online — detect unclosed tags, extra closing tags, mismatched nesting, deprecated tags, obsolete attributes, duplicate IDs and accessibility issues. Auto-fix, tag tree, sandboxed preview. 100% private, no signup.

It runs 100% in your browser — your files never leave your device, no signup is required, and there are no usage limits or watermarks.

How to Use the HTML Tag Checker

  1. Paste, drop, or type your input into the tool above.
  2. Adjust the options to match what you need.
  3. Preview the result before copying or downloading.
  4. Everything is generated on your device — nothing is uploaded.

Why Use Our HTML Tag Checker?

  • 100% private — files never leave your device.
  • No signup, no email, no limits — completely free.
  • Works on any device — desktop, tablet, mobile.
  • No watermarks, no hidden fees.

Frequently Asked Questions

Is the HTML Tag Checker free?

Yes — completely free with no signup, no email, and no usage limits.

Are my files uploaded to a server?

No. Everything runs in your browser using JavaScript. Your files never leave your device.

Do I need to install anything?

No — just open this page in any modern browser (Chrome, Firefox, Safari, Edge).

Does the HTML Tag Checker work on mobile?

Yes, on iOS and Android browsers. Very large files are best handled on desktop.

Is there a file size limit?

No hard limits. Files under 100 MB work well on most devices.

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