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The Nu Html Checker vs Nu Html Checker: catalog facts
The Nu Html Checker	Nu Html Checker
Free Free and open source under the MIT license.	Free Free to use on the W3C website.
Windows, macOS, Linux, Self-hosted, Command line	Web
Checks HTML, CSS, and SVG Can run in scripts or as a self-hosted HTTP service	Checks against current HTML conformance rules rather than old doctypes Accepts a URL, a file upload or pasted markup
The portable JAR and WAR packages require Java 17 or later Deploying the WAR package requires a servlet container such as Tomcat	Checks one document at a time Behavior is described as experimental and subject to change
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.