Run server:

python manage.py runserver

Check the python modules are installed:

python3 -m django --version
grammarinator-generate --version

Reproduce

in grammars folder:

touch fuzzer/HTMLGenerator.py  # Otherwise it will not find the file
grammarinator-process HTMLLexer.g4 HTMLParser.g4 -o fuzzer  # Fill the file
grammarinator-generate HTMLCustomGenerator.HTMLCustomGenerator -r htmlDocument -d 20 -o examples/test_%d.html -n 10 --sys-path ./fuzzer/
cd examples 
python -m http.server # start the server