Helicon Books epub validator is more than just another epubcheck. From the website:
Helicon Books validator has many advantages over other EPUB validators. It uses the epubcheck software from the IDPF but adds many additional tests.
First of all we don’t just take epubcheck output and display it, we display an easier to read and easier to understand error messages. Some error messages have long text, we display only the important part and let you read the rest only if you want (by clicking a ‘read more’ link).
Contact us if you need more information about any error, or if you want to learn more about EPUB3 and how to make it better.
In addition to validating the EPUB file with epubcheck, we are doing another phase of testing using a script we developed that checks the file for more warnings. This will be displayed separately as “Additional warnings”
We also test files according to guidelines used by Apple and Kobo when they accept books for distribution on their stores.
We check all mimetypes and produce a warning if we feel this may be the wrong mimetype for a file.
We also test the Table Of Contents file (for EPUB3 only) and provide warnings if the same file is linked twice. This is OK by the standard, but in many cases is not exactly what you wanted. We also check the titles for each chapter and make sure you don’t repeat titles. Again this is Ok by the standard, but we have seen several books that had mistakenly added the same file twice or gave two different titles to one content file.
We keep adding more and more tests to the validator and would love to hear your feedback.
So do not hesitate to contact us.
You can use the validator HERE.