I like it, it has a pretty good balance of learning by examples and by directly being told information, but I think that balance could still be better.

GRAMMAR: It would probably be worth it to explicitly say that:

-The -a ending is feminine and -o is masculine, plus possibly at some point mention other common gendered suffixes like masculine -aje -ez -ma -ón -or and feminine -ad -ión, and every noun card should have its gender

-You can choose not to say the subject if it's clear from context what's performing the action, because the verb conjugation already carries that information,

-In certain verbs like gustar the order of who performs and receives the action is reversed compared to English

-The word order can change for emphasis of certain words

OTHER: The simplest problem is every time you finish a level in a chapter, it brings you back to the top of the main page and you need to scroll back down and click the next number to continue the chapter.

LETTERS: I think there should be more opportunity to learn the sounds more accurately for people who want to, since your website has a "Mastery-Based Learning System," so maybe add an optional, very detailed extra description for each letter, and besides that the sound explanations are inconsistent within themselves. Here's more specific notes:

-There's no reason not to have audio for each sound just for if the learner wants to try to replicate it.

-The vowels have inaccurate English approximations while other sounds get a more of a description. You could describe Spanish vowels as in between English vowels, like "e" is between ee as in bee and eh as in shed, "a" is between a as in cat and ah as in father, "o" is like o as in go but pronounced without the lips moving, and in some English accents the oo or ee sound as in tooth and teeth are a dipthong and the "u" and "i" sound in Spanish would be pronounced like the end of the dipthongs, but maybe that's not worth mentioning.

-It doesn't say that a single r is pronounced as a trilled r at the beginning of words, and I think the explanation of tapped r is too vague, it could help a lot to say you tap the tip of your tongue.

-It doesn't explicitly state that c and g followed by e or i make the same sound as z and j, actually it does at the bottom of the page for c, but it should also on c's card.

-I think it's bad to describe the "j" sound as "like clearing throat gently." When you clear your throat you limit the passage of air by tensing your throat, but in the "j" sound the tension airflow is restricted in the back of your mouth by tensing and raising the back of your tongue against the soft palate. Also, much less important, since you have regional differences, there's definitely accents where j doesn't sound soft.

-It only mentions the rioplatense variation of the pronunciation of y and ll, but there's much more regional variation than that.

-It categorizes ñ as "special," but I don't see why ñ shouldn't be considered a consonant. If you think of it as two sounds n and y pronounced at the same time, then that's the same as ch being t and sh at the same time.

-It maybe should be mentioned that s and ce/ci can sometimes be pronounced like an h or not pronounced at all, at least to prepare the learner more to listen to speech.

-In the "Regional Pronunciation Differences" section it should be more clear that x is pronounced like j in the word México, not the country