Display one form field at a time for an economical layout
Preferably not when asking for username and password, since that messes with password manager autofill.
And yet more sites do it, even on desktop. As far as I can tell, most of them are also doing it in a way that breaks security by validating the username before asking for the password.
I know that single sign-on can be integrated that way.
For example, let’s say you work at Wheezecakes Inc. and want to log into your programming.dev account. Then you’d type your e-mail address, jadero@wheezecakes.com, into the username field and hit enter.
The webpage sends that to the server, which realizes that you’re a Wheezecakes employee, so it redirects you to login.wheezecakes.com or whatever SSO provider is in use, you log in there (or ideally already have a login cookie), and then programming.dev just gets told that, yeah, you’re authenticated to login.
So, while it’s obviously possible that webpages genuinely do this wrong, you’re probably seeing such SSO integration and they’re not actually validating the username ahead of time.