This is a simplified version of https://github.com/oasislabs/parcel-examples/tree/latest/login-with-oasis-frontend. That's documented at https://docs.oasislabs.com/parcel/latest/selected-topics/login-with-oasis.html.
All relevant code is in ./docs, and that is deployed to https://oasislabs.github.io/parcel-static-example-login-with-oasis-frontend/.