postResult method is used to send back an answer to the app that made the activity request.
Warning: As there is no formal contract between apps about the type of object to pass as answer, app makers should carefully handle the object they send and retrieve through activities.
Web Activities is not part of any specification. However, it has some overlap with the proposed Web Intents specification. Mozilla actually proposed Web Activities as a counter proposal to Web Intents. For more information about this, see discussion on the Web Intents Task Force ML.