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W3C Markup Validation Service vs HTMLProofer: catalog facts
W3C Markup Validation Service	HTMLProofer
Free A free service run by the W3C.	Free Free and open source.
Web	Windows, macOS, Linux, Command line
Validate by URL, file upload or direct input Choice of character encoding and document type	Checks generated output rather than source templates Works well in CI for static sites
HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker	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.