The main landing page is https://letterspractice.com. The submission link is an annotated view of a live study session, showing some of how the engine surfaces content, and how user-interactions feed back into the engine.
The slightly longer story is that (like many around here) I've had a long interest in Anki and SRS generally, and been frustrated at the gap between its effectiveness (high) and its success with respect to penetration into the mainstream (low).
So: many years ago I started playing with a web based framework ( https://github.com/patched-network/vue-skuilder ) for SRS based apps with a broader interaction model than front/back and self-grading. Seeking to apply the spacing paradigm to more interactive content types - skills rather than straight knowledge. Here is a brief demo of midi based ear training courseware I had made: https://www.youtube.com/watch?v=a6tvHMvF8Mo. I could write a lot about the framework, but the brief version of the ++ in SRS++ is pluggable pedagogy: applying Elo skill ladders, knowledge graphs, or other more custom structured discovery and evaluation to collections of content supported by an SRS core.
Some time later, I had a kid. I began using my own framework at a point to teach letter-sound correspondences, and word discrimination, and eventually spelling. It worked extremely well, with usage capped at n minute sessions whenever my kid was n years old.
Later still, I learned that premium edtech in the early literacy category sells at $500/mo. I'm sure the software is good, but also that it's out of reach for most families.
Knowing the efficacy (and compute efficiency!) of my own hodge-podge solution, I decided to smooth its edges, improve the decor, and make a broadly accessible product out of it (free month, $20 lifetime usage fee, good fallback to static-site for when I'm hit by bus).
So here we are!
(Taking all suggestions - entirely new at the B2C thing.)