You want to create *printable* flash cards? You don't want to use online flash cards?
If you are after printable ones, then why do you need flip between front and back? Just allow <textarea>s for both front and back at the same time, as (for example) that second site you posted does.
Even if you want to *present* the flash cards online, why does the person creating them need to be able to flip? Seems to me that would be less convenient than just giving them side by side textareas where they put in the front on the left and the back on the right.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.