I tried to create a button to trigger Geolocation API, but at least in Chrome it failed.
Geolocation access has been blocked because of a Feature Policy applied to the current document. See Sign in - Google Accounts for more details.
Indeed the iframe in Observable page has no allow="geolocation"
as suggested in the linked note.
Please, see this example. I’m using Chome 64.0.3282.119 (Build) built on Debian 9.3, running on Debian buster/sid (a 64 bit).