"If someone click it, it should asked the person if he is ready to downvote the post..."
I'm gonna have to respectfully disagree here, unless upvotes also have the same confirmation requirement. Downvotes ought to be just as easy as upvotes, in my humble opinion. While those confirmations could certainly help avoid so called fat finger errors, they can also discourage users from actually fully submitting their vote. Votes are amendable a good number of times if you change your mind after all.
I personally as a newbie was discouraged from downvoting on a number of occasions by required confirmations on other interfaces. It just kinda makes it seem scary to a new user, like you shouldn't be doing it or need some specific justification above and beyond what's required for upvotes.