I've done both CheerpJ and TeaVM. CheerpJ is awesome - it's a complete port of the JVM to JS and apps often work with no modification. However, the app + runtime is about 75mb (uncompressed) and can take minutes to load from a cold start.

This post links to the TeaVM version. TeaVM is also awesome - but it has a reduced runtime, conditional compilation and better built-in DOM support. The whole app + runtime is 1.2mb (compressed) and loads in seconds (instantly once cached).