How it works
How to use the HTML Validator
Paste HTML
A full document or fragment.
Review the issues
Each one is listed with its rule, line and column, and underlined in the editor.
Fix and re-check
Validation runs as you type.
What is checked
Validation uses html-validate’s recommended rules: document structure, nesting, duplicate IDs, deprecated elements and attributes, form labels, image alt text, button types and more.
Built for developers
Why developers use it
Private by design
Runs 100% in your browser. Nothing you paste is uploaded, logged or stored on a server.
Instant results
Output updates as you type, with precise error locations when something is wrong.
Works offline
Once loaded, the tool keeps working without a connection. Install it as an app if you like.
Free, no sign-up
No accounts, no limits, no watermarks.
FAQ
Frequently asked questions
Is this the W3C validator?
No — it is an offline validator with similar coverage plus accessibility checks, so your HTML never leaves the browser.
Is my data uploaded anywhere?
No. Everything runs locally in your browser — nothing you paste or open is sent to a server, logged or stored. The tool keeps working offline once the page has loaded.
Keep going
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