Two remarks : it would be nice for the divs not to move while using it. I'm not sure why but when trying to keep up the pace I was always distracted by something moving on the page. I think it is beacause your divs below with % of appearance/success was moving everything when going from a 1 digit % to a two digits?

Also, when seeing an hiragana and thinking it is a one letter such as "u" instead of "mi" the system does not really detect my "u" as wrong because it only has one character and expects two. So I can change my answer and get the correct one.

+ I think it would be good to allow alternative transliterations. E.g., "tu" for つ, "si" for し, "ti" for ち, etc.

+ It would be good to support composite/modified characters as well, stuff like しょ ("sho"), ちゅ ("chu"), じ ("ji"), じゅ ("ju"), が ("ga"), ぱ ("pa"), etc, etc, etc.

I'm just taking japanese for the first time, but I swear it's "yo".