Compare software

AChecker vs W3C Markup Validation Service: catalog facts
AChecker	W3C Markup Validation Service
Free Single checks are free; automatic monitoring is a paid ACHECKS subscription.	Free A free service run by the W3C.
Web	Web
Checks by URL, file upload or pasted HTML Choice of several guideline sets and levels	Validate by URL, file upload or direct input Choice of character encoding and document type
Guidelines stop at WCAG 2.0 Older interface with limited explanation of fixes	HTML5 validation is marked experimental Modern HTML is better served by the separate Nu Html Checker
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.