HTML Validator
Check your HTML for syntax errors, missing attributes, and W3C compliance issues — all in your browser with line-by-line error reporting.
Get notified the day we launch. No spam, ever.
What to expect
Built for developers
Validate HTML markup for syntax errors, missing alt attributes, deprecated tags, and W3C compliance issues with detailed reports.
Runs client-side. Your data never leaves your browser.
No ads, no trackers, no popups. Pure focus.
Paste code or upload files. Both modes supported.
Share output with teammates via a permanent link.
Adapts to your system preference automatically.
Fully responsive. Works on any screen size.
About this tool
OpenFormatter's HTML Validator checks your HTML markup for errors and compliance with W3C standards. Supports HTML5, detects unclosed tags, missing required attributes, deprecated elements, and nesting errors. Provides line-by-line error reporting with severity levels (error, warning, info). All processing happens in your browser.