I don't do this myself, but I did consider doing it, because to some degree it simplifies work flow in the login process. By way of background I should point o

I don't do this myself, but I did consider doing it, because to some degree it simplifies work flow in the login process. By way of background I should point out that logins can be a Lot more complicated than just "enter login and password" and critically the complexity may be different per user. For example some users have 2FA turned on, so we need to collect user name, password And say SMS code. This is the very tip of the iceberg. So identifying the user first can make it easier to then determine which path to follow. Ultimately I didn't go this route because AJAX can be used to get the user-name when entered. However it can then get complicated when the user name and password are entered and the login button clicked at the same time (like by a password manager). So multi-step input is easier to code.