I'm using the HTML5 doctype.
There are two things here, autocomplete and autofill. The thing you can add to input fields is autocomplete, which despite the name of this thread, is not what I'm after. The thing I'm after, which is also sometimes called autocomplete, is really autofill. Its when login and password fields get filled in when a page loads (vs autocomplete which just suggests things as you type). And the hardest part of autofill, which seems to be initiating the browser actually asking you if you want it to remember your password.