To run the demo, you can open the file demo.html from the browser "File/Open..." menu.
Another option is to start the built-in Python HTTP server by
python -m http.server
The default port is 8000. To specify another port:
python -m http.server 8080
Then load http://localhost:<port>/demo.html in the browser address bar.
For more information please visit http://brython.info.