HTML Validator
Free HTML validator. Check for syntax errors, unclosed tags, and invalid nesting instantly.
How HTML Validator Works
An HTML validator checks your markup against W3C standards, catching syntax errors, unclosed tags, attribute mismatches, and invalid nesting before they cause browser rendering issues or accessibility failures. Unlike server-side validators, this tool runs entirely in your browser — your code never leaves your device. Paste any HTML snippet or full document above to check it instantly.
Common Errors Caught
| Label | Meaning |
|---|---|
| Unclosed tags | Most common: <div>, <p>, <span> |
| Missing alt attributes | On <img> elements — critical for accessibility |
| Duplicate id attributes | IDs must be unique per page |
| Deprecated elements | <center>, <font>, <b> overuse instead of semantic tags |
| Invalid nesting | Block/inline elements incorrectly nested |
Frequently Asked Questions
What does an HTML validator check?
An HTML validator checks for: (1) Syntax errors like unclosed tags, (2) Attribute mismatches (e.g., invalid attribute values), (3) Invalid nesting of block and inline elements, (4) Missing required attributes (e.g., alt on images), (5) Deprecated HTML4 elements, and (6) W3C compliance issues.
Is this W3C compliant?
This validator checks against W3C HTML5 standards. It reports all deviations from the official W3C spec, helping you produce valid, cross-browser-compatible markup.
Why should I validate HTML?
Valid HTML improves SEO (search engines parse valid markup better), accessibility (screen readers rely on semantic HTML), and browser compatibility (prevents unexpected rendering bugs). It also helps prevent JavaScript errors caused by malformed DOM.
Is my HTML code stored?
No. All validation happens client-side in your browser. Your HTML code never leaves your device — we never see, store, or transmit it.
Does it support HTML5?
Yes. This validator fully supports HTML5 and checks against modern W3C standards. It recognizes all valid HTML5 elements, attributes, and ARIA roles.
About This Tool
Built by the Calcyo team and last updated June 2026. All calculations follow industry-standard methodology. No data leaves your browser — calculations run entirely client-side using JavaScript. If you spot an error in the formula or benchmark data, email us at support@calcyo.xyz.