Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
This version uses the new JSON style validation service on validator.nu
Your html source is posted to https://validator.nu/
Unfortunately the new service does not return error codes like it did before so the filtering of certain error types has been disabled.
You can edit it by typing "about:config" in Firefox's address bar, then search for extensions.validator.address and edit as you wish.
Note that validator.w3.org is always used as a failover.