Anki flash cards (floss) do use spaced time learning algorithms that are also easily adjustable to personal preference. Anki has native mobile/desktop, cli and web clients. The big community also provides shared decks and vast plugin ecosystem like auto text-to-speech and imagery. I haven't seen any of these tools doing anything that anki doesn't — why not contribute to it instead? https://github.com/ankitects/anki
Because Anki's UI could do with a lot of improvement, and it is probably eaiser to separate out the algorithms, the data and the UI.
If anything new Anki UI project would be more impressive and clearly in demand compared to yet another flash card web app.