Offline discussion sugggests that the API should support some retry mechanism otherwise PendingBeacon is too unreliable.

1. bool field option {retry: true}: this option let the browser decide how to retry the failed beacon request.

2. number field option {retries: 10}: this option let the user specify how many times the browser should retry the failed request. However, it might be very difficult to implement as: by default the beacons are sent out on page discard, and there really isn't much time left for a page to retry that many number of times, unless we are willing to let browser queue a failed request longer enough (privacy concern).

Note to @fergald : Looking into the spec of Reporting API, it doesn't really support retry mechanism.

We don’t specify any retry mechanism here for failed reports. We may want to add one here, or provide some indication that the delivery failed.