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W3C Markup Validation Service vs The Nu Html Checker: catalog facts
W3C Markup Validation Service	The Nu Html Checker
Free A free service run by the W3C.	Free Free and open source under the MIT license.
Web	Windows, macOS, Linux, Self-hosted, Command line
Validate by URL, file upload or direct input Choice of character encoding and document type	Checks HTML, CSS, and SVG Can run in scripts or as a self-hosted HTTP service
HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker	The portable JAR and WAR packages require Java 17 or later Deploying the WAR package requires a servlet container such as Tomcat
Web application	Downloadable app, Self-hosted
Closed source	Open source
License not stated	License: MIT
No account needed	Not stated if an account is 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 October 8, 2026

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