Some constructive criticism:

1. For picking the kana answers, using the keyboard key is better than numbers. When you actually type an え, you type 'e', so it's a useful mapping to learn in terms of how IME works.

2. For vocabulary, there should be an option to turn off romaji in favor of kana only. No explanation needed I think

3. The vocab quiz, between kanji and just an english word, is an anti-pattern in my opinion. Recognizing the meaning if vocab in a full japanese sentence is a much better basic quiz, especially since not all words have 1-1 mappings. It also doesn't quiz on the reading, which seems weird. Also, an easy example of something confusing there is that 辺 is 'area', but if I see 'area' my first thought is 面積 (like the area of a triangle), while 辺 would be edge in that context... and my second thought is 地域, like "the area of the country I grew up in". I think 辺 is maybe 4th or 5th for 'area', and that's just because 'area' is a broad english word. My point is, quizzing vocab -> english word, without reading, without an example sentence, is a recipe to confuse learner's brains.

4. Same complaints as vocab for the kanji quiz, but moreso since kanji's meaning is more abstract.

The beautiful aesthetic and open-source way to learn Japanese is to make Anki flash cards, and customize the cards using html (which it already supports).

This entire site could have been anki decks, and then it would have had spaced repetition for free, and users could even more easily edit things to suit themselves ad add to it.

Absolutely.

As a long-term Japanese user, I won't even consider a learning system for Japanese unless it has #2, and the other 3 are highly, highly desirable. So much so, that I can't imagine picking a system that doesn't understand why they're better, since so many other systems already exist that do.