Sigil 0.6 is out for some time and this version allows to switch HTMLTidy of so (at last!) it is possible to insert html5 tags (like video and audio).
Sigil is better for epub creation (by hand) than Calibre because it does not insert it's own css.
At the moment this is the best software available: I have tried Jutoh, but development seems to have stopped (and the interface is ...not standard - to say the least!) and conversion utilities like eCub (and Calibre), Legend Maker, eBookBinder, MePub and a lot of Python scripts (I even wrote one myself) but none is as easy to use as Sigil.
So: at this moment this is the best option available - even as it is slow on saving projects because it uses an already packed epub file format. Recommended!