There are a couple of related issues touching on aspects of supporting background geolocation. Let's use this one to track background geofencing specifically.

For reference, @mkruisselbrink sent a proposed API to the Geolocation WG mailing list back in 2014. That seems like a reasonable place to start if someone wants to pick up this work.

Note that at time of writing no implementers appear to be interested in developing an API for background geolocation or geofencing and so any specification work done in this group will be purely speculative.