IMO we should clarify the following:

1) Add a note (maybe just informative?) saying user name / password from open() method will only be sent to a site if it first uses a 401 response to indicate that authentication is required.

2) Figure out what should happen if a script calls open() with user name/password arguments, then sets an Authorize header with setRequestHeader(). Which wins? Will it depend on whether the site says 401 or not?

(IMO: setRequestHeader() should win if this is compatible with implementations, simplifies things. Whether or not there is a 401 response should make no difference. Hope that's sufficiently aligned with implementations..)

3) I assume that if setRequestHeader() adds an Authorize header, it's sent to the server whether or not a 401 request has been returned. Perhaps this should also be noted.