It's strange to see a mobile page in a desktop browser, but it wouldn't be the first time when Google ported mobile sites or apps to the desktop. Most of the new features are added to the mobile sites first.
I haven't seen the new interface in a desktop browser, but the mobile sign-in page has an important issue: it doesn't allow browsers to autofill your password if you've previously signed in to a Google account. The sign-in page remembers all the accounts you've used to sign in, so you can quickly sign out and sign in to a different account just by choosing one of them.
{ Thanks, KH Subscribe and Arunraja. }