Related URL per sync

[jakearchibald]

This is beyond MVP I think, but interested to get thoughts on:

serviceWorkerRegistration.sync.register({
  tag: 'send-email',
  progressURL: '/outbox/'
});

…where progressURL (optional) would give the user details about the progress of the sync - the outbox in this case.

If the browser enumerates pending/complete syncs, they could link to this URL.

I feel this may be too implementation specific, but keep to hear Mozilla's thoughts.