Webapps have been dealing with this for years, it's not anywhere as painful as you are making it seem. Pick a base version to support, use feature detection to add anything new, and you're golden. The existing browser APIs cover 99% of all apps' needs already, and when you're doing this you can always hand-off things to the native layer as needed.

Electron sets a very unrealistic eternal-greenfield-development standard that is nothing like developing for the web.