I was working on the calibre 4.0 post this morning when I discovered a new calibre feature. Well, it's not new so much as I hadn't noticed it before.
It seems that in May 2018 calibre gained the ability to make Epub 3 files. This should really come as no surprise (the new version of Epub was bound to replace the old eventually) but I for one did not know, and I thought it was worth bringing to your attention.
You can find the conversion option in the "Epub output" tab in the conversion menu. It's rather well hidden, yes, which is why I expect that no one is using this option.
While we are on the topic, what calibre feature do you think everyone should be using?
P.S. That reminds me, I should probably update my post on cool tricks for calibre.