From the description, it sounds like perhaps they transpile Java Bytecode to Javascript?
> Based on CheerpJ, a solution to convert Java applications to JavaScript, the Applet Viewer extension compiles the Java applet on the fly, without relying on any remote server.
Yes, from the FAQ: "The CheerpJ compiler, based on LLVM/Clang, as well as on parts of Cheerp, converts Java bytecode into JavaScript, without requiring the Java source. CheerpJ can be invoked on whole Java archives (.jar) or on single .class files, and generates a .jar.js (or .js) output. "
We've seen this plenty with GWT and the likes, but this one is self-hosted in the browser to support dynamic class loading. As with Emscripten, the real struggle is the stdlib, not the translation. As an alternative, I would recommend people look at TeaVM[0].