> // Note the Turkish dotless i 'John@Gıthub.com'.toLowerCase() === 'John@Github.com'.toLowerCase()

I'm not sure this example is correct. The dotless ı is already lower cased, so the comparison above should yield false. Maybe the author was thinking about upper case dotted "İ", which becomes regular dotted "i" when lower cased.

So what could happen is that an user enter "JOHN@GİTHUB.COM" as email, and then the email is sent to "john@github.com" .