Hello, I implemented the mustache specification in Elm over at https://github.com/emmabastas/elm-mustache/

It supports (as in, passes the tests in mustache/spec) every non-optional feature except for partials (would it make sense to have this in a niche browser language without file access?) and it doesn't preserve \r\n -style line endings (for no particular reason, should fix that at some point).

I know that this Elm implementation is already linked from the website. However, that package is more limited (supports only normal variables and boolean sections, and does not handle standalone tags and other edge cases) and has since been archived.