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W3C Markup Validation Service vs htmltest: catalog facts
W3C Markup Validation Service	htmltest
Free A free service run by the W3C.	Free Free and open source.
Web	Command line
Validate by URL, file upload or direct input Choice of character encoding and document type	No Ruby environment needed, unlike html-proofer Single Go program that also runs in Docker
HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker	Works on generated files, not on live dynamic sites Command line only, with no graphical interface
Web application	Downloadable app
Closed source	Open source
License not stated	License not stated
No account needed	No account needed
Needs internet	Works offline
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked September 24, 2026

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