[DiapDealer]

If an EPUB3 open in Sigil generates EpubCheck errors, I can assure you that those errors are "real problems". EPUB3s do not inherently generate any more EpubCheck errors than EPUB2s do.

And yes, we very much would like to know of any "weird" Sigil behavior when working with EPUB3s. I work nearly exclusively with EPUB3, and it works as it's designed for me. If there are problems, we'd like a chance to fix them.