The cross-widget messaging feature works within the browser, but due to cross-origin browser security policies, the web page and script files (which may be found at
https://github.com/aml-development/ozp-iwc) used to start the shared service worker must be hosted on a single domain. The back-end support (other than for simply hosting the page and scripts) is optional, but enables features such as persistent data storage. If you are only developing locally, you could host the IWC peer file and scripts on the same server you are hosting your widgets.