I was a bit annoyed that applets and JARs cannot be made to run in the browser... so I tried (with some success) to use cheerpj.
But more recently, I decided, just for fun, to re-implemented (some main parts of) Greenfoot in javascript
I was surprised to find that JavaScript classes and reflection mechanisms are now actually usable. So, even if the language is loosely typed and interpreted, it was possible to implement many of the central GF features.
If anybody wants to have a go at Greenfoot_JS, check it out here at my personal site: andval.net:
the link
Feel free to tell me how (terrible) you find it! :D