epubcheck Error

Hi all,

I am trying to publish my first book. I wrote it in HTML and converted it to epub using Calibre. On the Google Play Store, I received the following error on about half of my splits and I have no idea how to address them:

Error while parsing file: attribute "class" not allowed here; expected attribute "dir", "lang", "version" or "xml:lang"

Any thoughts would be greatly appreciated.