[GeoffR]

That will really mess up books where the publisher uses <p> for headings etc., and won't have much effect on books where the publish

Edit: One specific problem likely to be caused by margin: 0em !important; is the loss of scene breaks of the sort where the new scene is indicated by a top margin on the opening paragraph.

Edit: My experience is that each ebook is constructed differently, there is nothing even close to a standard even with books produced by the same publisher, and so any attempt to apply a one-size-fits-all algorithm is a waste of time. I just edit each one manually, and if that sounds like a lot of work then think of it in terms of the time spent reading the book: a few minutes editing (for 98% of books, a small minority take longer) vs. many hours reading and re-reading.