Assuming progressive login is separate in two distinct pages.
Are there two seperate entries stored in the browser (login name and password) or does the browser still understand that this is a single login with a login name and a password?
Browsers may become smarter and detect this kind of login page(s) but if they don't then there will be two entries in your auto-complete data: one to fill the username field and another to fill the password field.
Do browsers have special features to handle progressive disclosure during login?
They don't. It means that they can't nicely handle multiple accounts.
As a developer: Do I have to cheat the browser to tell this is one login form?
Yes, do not use distinct pages. If you split login in two pages then you will probably break password managers and it is a major inconvenience you should avoid at all costs.
Login is still one page
Are there two seperate entries stored in the browser (login name and password) or does the browser still understand that this is a single login with a login name and a password?
No, they're handled as usual.
Do browsers have special features to handle progressive disclosure during login?
No, they do not need to because everything works as before.
As a developer: Do I have to cheat the browser to tell this is one login form?
You do not have to cheat the browser. Take a look on how it's currently implemented for example by Google. Username and password are still there together, on the same <form>. When you click on Next the <div> that contains <input type=text> and relevant text is hidden and password becomes visible.
Do you have to do anything else? Nothing more than usual, properly fill aria-, autocomplete and spellcheck attributes and modern browsers will grant a good user experience.
