[DNSB]

Or if you are enthused, you can add the EpubCheck plugin to Sigil or calibre's editor. For most places, if it doesn't pass epubcheck, they won't even try to publish it. If you don't want to use Sigil or calibre, you can run epubcheck from a command line though I generally reserve that for pre-processing a mass of epubs.

For the last few years it's been better but the memory of how many "professionally" produced epubs failed epubcheck, often in a rather spectacular display of error messages still drives my lack of enthusiasm for most publishing houses ability to produce a half decently formatted ebook.