Free Online HTML Validator
Validate HTML syntax with detailed error messages and line numbers. Format HTML with proper indentation.
html Input HTML
Characters: 0
Lines: 0
Tags: -
code Output Result
Click Validate to check HTML syntax.
Formatted or preview output will appear here.
Key Features
Instant Validation
Check HTML for unclosed tags, duplicate IDs, missing DOCTYPE, and improper nesting.
Error Details
Each error and warning includes the exact line number where the issue was detected.
HTML Formatting
Beautify HTML with automatic 2-space indentation for clean, readable structure.
Privacy Protected
All processing happens locally in your browser. No data ever leaves your device.
Frequently Asked Questions
Getting Started
What is an HTML validator?
The HTML Validator checks your HTML code against the HTML specification, identifying syntax errors like unclosed tags, improper nesting, duplicate IDs, missing DOCTYPE declarations, and deprecated elements. It uses the browser's native DOMParser and reports each issue with the exact line number.
Is this HTML validator free?
Yes. All tools are completely free and run locally in your browser. No signup, no limits, no hidden costs.
Does the tool store my data?
No. Your HTML data never leaves your browser. All processing happens locally on your device, ensuring complete privacy and security.
Features & Usage
Does this validator catch all W3C violations?
No, this is a client-side validator that catches well-formedness issues but does not validate against a specific DTD or check every nuance of the W3C spec like the official validator does. For strict standards compliance, pair this with the W3C Markup Validation Service.
Why does the browser render my page fine even though the validator shows errors?
Browsers are built to be tolerant of malformed HTML and apply error-correction algorithms. Each browser's error recovery differs slightly, which can produce inconsistent layouts. Validation eliminates this guesswork.
Can this tool validate HTML inside JavaScript template literals or JSX?
No, this validator expects plain HTML documents or fragments. If you paste JSX (React), Vue SFC templates, or HTML embedded in JavaScript strings, the parser will treat the surrounding code as invalid HTML and produce many false positives.
Can I validate a live URL instead of pasting code?
Not directly, but you can open the page in your browser, view source (Ctrl+U), copy the HTML, and paste it here. This approach also keeps your session credentials safe.