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HTMLProofer vs Nu Html Checker: catalog facts
HTMLProofer	Nu Html Checker
Free Free and open source.	Free Free to use on the W3C website.
Windows, macOS, Linux, Command line	Web
Checks generated output rather than source templates Works well in CI for static sites	Checks against current HTML conformance rules rather than old doctypes Accepts a URL, a file upload or pasted markup
Requires a Ruby environment Works on local HTML files rather than crawling live sites	Checks one document at a time Behavior is described as experimental and subject to change
Downloadable app	Web application
Open source	Closed source
License not stated	License not stated
No account needed	No account needed
Not stated if it works offline	Needs internet
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.