EPUB Validator

None, really. The best 'validator' is Thorium Reader; if your document proofs/previews/reads there, it's as valid as EPUB can get.

Some rely heavily on validators, but all they do is check code against a standard that is only loosely implemented out in the real world. A fully validated EPUB is not necessarily any more likely to read perfectly for every user on every reader than one that merely passes in Thorium. You can spend hours and much hair-tearing fixing minor validation errors that in the end simply don't matter.

The ONE place validation is useful is if you are concerned about alternate accessibility. Validation can help target glitches that visual readers will never notice but voice-readers and readers that accommodate users with other limitations might trip over—mostly internal structure flaws. But even then, if Thorium voice-reads the document properly, further technical validation isn't going to add much.

Validators are also more important to the community using shareware and other builder tools, where it's more common to make fundamental mistakes in assembling and linking the elements of an EPUB file. If you're exporting from ID, such mistakes are rare and usually evident in good ol' Thorium. 🙂

(The other useful validator is conversion to Kindle in Kindle Previewer, which is a different road but if KP tolerates it, it's "valid.")