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W3C Markup Validation Service vs HTML-validate: catalog facts
W3C Markup Validation Service	HTML-validate
Free A free service run by the W3C.	Free Free to use, with the source code published on GitLab.
Web	Command line
Validate by URL, file upload or direct input Choice of character encoding and document type	Validates entirely offline, so no markup is uploaded Handles HTML fragments inside Vue.js and AngularJS components
HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker	Command-line and tooling oriented, with no graphical interface Checks markup only, not CSS or accessibility in a browser
Web application	Downloadable app
Closed source	Closed 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 October 2, 2026

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