Use cases: 1) A screen reader that sees a form with a type=username and a password field. The screen reader could just ask "Log in to this site? [y/n]?". No further context would be needed. 2) UAs can more easily discover login forms and offer things such as Firefox's Account Manager [1] or a "remember me" feature 3) Currently autofill for usernames looks for something like id="username" or name="username". However on certain websites this fails. Furthermore some websites offer a "find other members" feature where you could type in a username. I've often seen these fields filled in automatically with my name. 4) I'm sure there are others....
The proposal: A type="username" is added to the input element. type="username" would MUST only be used for the name that is used to log in to the site. It MUST NOT be used for registration forms or anything else that requires a username. A form MAY have up to one (but not more) type="username" input field. [1] http://www.mozilla.com/en-US/firefox/accountmanager/
