Some discussion here: https://meta.discourse.org/t/tab-at-login-styling/183425/

This changes the tabindex of elements so the "Skip the password; email me a login link" link that shows under the username input doesn't interrupt the username -> password tab order that keyboard users and autofill extensions have come to expect.

Visually the "Skip the password; email me a login link" under the username input is the "right place, right time" for users that aren't navigating with their keyboard... so using tabindex is a compromise.