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The Nu Html Checker vs W3C Markup Validation Service: catalog facts
The Nu Html Checker	W3C Markup Validation Service
Free Free and open source under the MIT license.	Free A free service run by the W3C.
Windows, macOS, Linux, Self-hosted, Command line	Web
Checks HTML, CSS, and SVG Can run in scripts or as a self-hosted HTTP service	Validate by URL, file upload or direct input Choice of character encoding and document type
The portable JAR and WAR packages require Java 17 or later Deploying the WAR package requires a servlet container such as Tomcat	HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker
Downloadable app, Self-hosted	Web application
Open source	Closed source
License: MIT	License not stated
Not stated if an account is needed	No account needed
Not stated if it works offline	Needs internet
Official website Full listing and sources Checked October 8, 2026	Official website Full listing and sources Checked September 23, 2026

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