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W3C Markup Validation Service vs AChecker: catalog facts
W3C Markup Validation Service	AChecker
Free A free service run by the W3C.	Free Single checks are free; automatic monitoring is a paid ACHECKS subscription.
Web	Web
Validate by URL, file upload or direct input Choice of character encoding and document type	Checks by URL, file upload or pasted HTML Choice of several guideline sets and levels
HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker	Guidelines stop at WCAG 2.0 Older interface with limited explanation of fixes
Web application	Web application
Closed source	Closed source
License not stated	License not stated
No account needed	No account needed
Needs internet	Needs internet
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked September 23, 2026

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