Here a simple bit of JavaScript allows the form to be submitted only if there's something (in this case, anything at all) in both user name and password fields - if either is empty the submit button is greyed out and the form can't be submitted. (Added label tags and a bit of CSS make the form look a bit tidier too.)
Try submitting the form in various ways. If the cursor is in the User name field and you hit the Enter key, or if you click the Go button, everything behaves normally. If you clicked in the Password field on the way - maybe you typed something into it - you might have noticed the Typinator icon change, but it would change back when you clicked on either the User name field or the Go button. What happens, however, if you submit the form by hitting Enter when the cursor is in the Password field? Try it and see...