Is browser VM a good idea?

It will be great if we can use a single language on both server and client. Similar to JVM, if we have a builtin browser VM which compiles to byte code, and can use different languages it will be good. e.g. Use a python library written for browser VM in Java. Why should we invent new languages like DART, CofeeScript which in the end compiles to Javascript. Is Javascript the new byte code?