To use the W3C Markup Validation Service:
- Set your theme to "standard" or "standardwhite".
- Add a user with the username "w3cvalidator".
- Give that user all the roles and permissions that you want the validator to have. Easiest is just to make it an admin.
- (Optional) Turn debugging up a little in Administration > Server > Debugging. This will give you handy validation links in the footer.
- Click on the links to validate that page (or paste URLs directly). They should work now, because Moodle automatically detects the validator and treats it as the w3cvalidator user.
- Web developer extension
- Using Moodle How to use http://validator.w3.org/ without login? forum discussion
- Using Moodle xhtml-validation problem forum discussion