Push API and Service Workers

Hi,

I'm with the IE Platform team at Microsoft.  I joined the WebApps WG very recently.  I am looking into the Push API spec, and got some questions.  Hope to get help from experts in the WG.

The current API definition is based on an extension of the Service Workers.  I'd like to understand whether the Service Workers is a must-have dependency for all scenarios.  It seems some basic scenarios can be enabled without the Service Worker if the website can directly create a PushRegistrationManager.  I'm looking at Fig. 1 in the spec.  If the user agent can be the broker between the "Push client" and the webpage, it seems some generic actions can be defined without the Service Workers - for example immediately display a toast notification with the push message.

It is very likely I have missed some basic design principle behind the current API design.  It'd be great if someone could share insights on the scenarios and the normative dependency on the Service Workers.

All the Best, Shijun