Update: They got back to me. Here's their reply,
Unfortunately, the ePubs are still not able to process in our system due to the '.ui-button:hover' in the CSS.
For example in 9798996819126, these appear to be in the following CSS files:
jquery-ui.min.css
jquery-ui.structure.min.css
jquery-ui.theme.min.css
Our Development is still working on a fix, but we do not have an estimated date for when it will be fixed. I have asked for additional investigation regarding the books you mentioned that were published with those lines included in the CSS.
Can I remove those and still validate?