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