Calibre is truly powerful. I know I've just scratched the surface. It's like an onion!
I've been experimenting with removing unused CSS via the command line (ebook-polish -u). I didn't see any options in the Calibre manual that will trigger a listing of what was removed. Did I miss it somewhere?
I will explore that plug-in for removing those irritating empty <spans> (where do they come from and why? why? why?)