[mrmikel]

You have left off or never had when you imported, paragraph end tags or other end tag or end tags with no start tags, or anchor ends without anchor starts, the list goes on.

You can run it through flight crew to see the list or run it through epub check (that will give you many errors that aren't so important. You can also load it into calibre's editor if you have calibre installed and choose check book.

If you are bringing things in from Calibre, it won't have a doctype by design on K. Goyal's part. But you can tell it to let it fix things for you and it will add it.