Do note that if you need to target macOS 10 + Safari, WebP is not available.
WebP is available for macOS 11, even with overlapping versions of Safari, but Apple relies on the OS image library to render some images and they haven't backported WebP supported when they updated Safari.
Unfortunately yes. Currently it just checks whether the Pokemon Go login servers are accessible. I would like to add support for multiple regions, but as far as I can see doing so is non-trivial.