Logout is possible: The server has to send http status 401 if I recall correctly. The browser treats this as logout.
And well, my experience there is about 20 years old, but back then sending 401 wasn't enough. You had to send a different auth request, which then would pop up a new password dialog first. Indoubt 401 is enough today as well, as the browser can't distinguish whether a specific resource is restricted or whether the whole session should be invalidated.