Form of mental model of navigation – Exchange of user experience stack

If a user comes from a Windows-based software, how do you navigate through the entries with enter password and save for Ctrl + s, to browser-based software where the default navigation is done with the tab key and save with get in, What can I assume about the user's mental model?

If you are an advanced user who has filled Windows-based forms quickly with only the keyboard, and to put it extreme, with only numeric keys where you can type a number, press enter, type a number, press enter, and so on. Do we assume that the user expects the same type of behavior in the browser or should we assume that he uses the browser software as he has learned in other browser-based forms (such as, navigate with the tab and send with enter)?

Advanced user functions, keyboard shortcuts, etc. They are an excellent complement to the software of a browser, but when it conflicts with the default way in which the browser handles keyboard events, I think it is a very dangerous code and too easy to solve (with some exceptions, such as software of word processing like Google Docs … but the main point is to think about software based on forms).

So I am in a dilemma and I need different points of view.