Adding --emrun sounds like a good idea. 

emrun does work currently on any html page without modifications (even on non-emscripten-generated html files). Adding the postjs just enables stdout+stderr and exit(returncode) capture. I would not like to strictly fail if --emrun was not passed, since it will still run, but not just get those data. How about if I just add a warning "File does not seem to have been compiled with --emrun, stdout+stderr capture might not work." at emrun startup if the file was not --emrun-enabled?