I wrote that in the twitch chat yesterday already but I thought for completion, I'l post it here again.
On macOS Sierra 10.12.1 and Safari 10.0.1 it is not possible for me to register or login. It works with Firefox though (even though I didn't get an email confirmation as many others). I thought maybe registration worked in Safari with no confirmation (as it did in Firefox). To find out I "created" an account in Safari and tried to login on Firefox but it said that the account doesn't exist. So no registration took place => Registration in Safari doesn't work for me.
To test the login thing I entered Name and Password of the account I created in Firefox and tried to use it in Safari but nothing happened => Login in Safari doesn't work.
I hope the explanation is helpful somehow.
Thanks for the effort, it already looks great!