Srsly is spaced-repetition using the Hypothes.is annotation platform. See this blog post for more details.
Srsly is hosted at https://srsly.netlify.app/, you are free to use it.
Disclaimer: note that using the service means me (the service owner) is able to see things, such as your Hypothesis token and your flashcards. If that bothers you then don't use it.