it makes connecting to localhost as easy as http://0:8080/ (for port 8080, but omit for port 80).

The thing is that it’s not supposed to work, so it’s essentially relying on undefined behaviour. Typing [::1]:8080 is nearly as easy.

skimming through these PRs, at least for WebKit, I don’t see tests for shorthand IPs like 0 (and no Apple device to test with). What are the chances they missed those…?

I haven’t seen the PRs, but IP comparison should really be using the binary form of the IPv4 address (a 32-bit number), not the human-friendly form.