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