Oh hey there. Website fonts are one of those things that nobody except the website creators usually care about, because most fonts are either ones basically everyone has, or included in the website itself, but sometimes weird things like this can come up, because WaniKani does not include their fonts in the website.
Hiragino is included on Mac and iOS devices, but if you’re using anything else, it’s possible some things may not display the intended way unless you go out and find the font for yourself and install it on your computer. You can consider that the “proper” look of all the kanji on WaniKani, if that matters to you at all.
Here’s my original post for reference: